Europe of Five Empires

They entirely lack the cultural and political background for that kind of antagonism to flourish. Because of their vast size and diversity, all the empires picked the strategy of adopting a common imperial language (Latin, Greek, Germanic Anglo-Norse, different variants of East Slav) and making everyone of import speak it, with remarkable success. The nobility, clergy, bureaucracy, officer corps, and bourgeousie universally speak them, and everybody knows one of the necessary steps to raise above the station of peasant or laborer or get a basic education is to get fluent in them. The other languages do exist as peasant dialects but they have no prestige, no literary representation, and no standardization whatsoever. Literacy involves mastering the imperial languages, and all literature and printing is in them. The political and cultural atmosphere makes a literary revival of the ethnic languages of the kind that happened in our 19th century exceedingly unlikely, so in all likelihood industrialization and public education shall bring universal fluency in the imperial languages and extinction or marginalization of the rest.

The neo-Roman, universalist European super-culture the empires share frowns on ethnic division as an hallmark of barbarism, backwardness, anarchy, and paganism. They lack any real memory of ethnic proto-nation states flourishing and being a positive precedent since the empires took root and spread too early and effectively for them to stand out. As the Europeans remember their past, it is a broad sequence of Bronze/Iron Age Chaos-Roman Order-Dark Age Chaos-fivefold Imperial Order, so there is little room for nostalgia of independent tribal or barbaric kingdoms. Because of its very premises, this dominant European mindset shuns ethnic prejudice: if you are a loyal imperial subject and speak the imperial language, you are treated as an equal and with the appropriate amount of luck and effort you (or your children) can rise to join the elites regardless of your birthplace or your ancestors. Non-Whites face the additional ordeal of adopting European culture and converting to Christianity, and their path to social climbing is all the more difficult since they almost inevitably start at the bottom after colonization, but if they do, even skin color is not a unsurmountable barrier to the talented and the lucky. So nationalism is not really useful as a rallying banner to claim equality.

If they look outside of their continent, the Europeans notice all the other civilizations that can stand up to them as peers of similar power and sophistication are empires as vast, centralized, ethnically diverse, and culturally united as themselves, and all the other cultures that may be closer to the notion of ethnic polities are invariably weaker, less organized, and less advanced than themselves, and a victim of their colonial expansion. The closest the Europeans get to identify a positive notion of 'national' identity beyond a civic ideal is the broad dominant cultures that stand at the basis of the various empires, such as Latin for the HRE, Greek for the ERE, Germanic for the NSE, Slav for LRE/Muscovy, Mandarin for China, Hindi for India, and hybrid 'Japorean' for Japan-Korea. Early and extensive contact between the European and Asian empires drove both sides to mutually reinforce this shared global vision of history and culture, so both sides are driven to see their mirror image in the other, and regard the imperial path as the only true one and God's plan to bring order, peace, and good government to the world. Add to this that this high degree of cultural and political unity considerably enhances global trade and cultural exchanges, not to mention the coming effects of indistrialization, and you may see how it becomes a self-reinforcing pattern. The occasional rabble-rouser or scholar arguing for a revival of long-suppressed national identities invariably gets swept away by the tide, suppressed or ignored. Romantic nationalism stands more or less the same chances of appeal and success in this world as Amish luddism.

They do. Early and extensive contact with Europe drove the advanced Asian civilizations to shake off isolationist complacence, pursue imperial centralization where it was lacking, and open to the world to avoid colonization. Notable effects of this include: China avoided Confucian stagnation and decline (they may or may not have experienced the equivalent of the Ming-Manchu dynastic shift but probably the Han remained in charge), northern-central India grew into a fairly stable and united empire that pursued the establishment of a common proto-Hindi language (decline and extinction of discredited Islam avoided religious division and considerably helped the equivalent of Moghul or Maratha empires to get entrenched), Japan avoided Tokugawa isolationism and fused with Korea to form an expansionist, trade-oriented hybrid 'Japorean' empire and culture that colonized Greater Manchuria, Kamchatka, Taiwan, Hainan, the Philippines, and Micronesia (plus of course the usual Ryukyu and Kurili islands). In comparison, the polities of the Malay Archipelago were too weak, disorganized, and exposed to European (or Japanese) colonialism to follow the example of their neighbors. It remains an open question if Indochina shall follow one pattern or the other.

My best guess is the new rulers of this united empire pursued interbreeding and social mingling of Japanese and Korean royal families and nobilities until they stopped being distinct entities. Likely analogues include the Yamato imperial family and the Fujiwara regent clan extensively interbreeding during the Heian era, the various types of Japanese aristocracy fusing in a unitary peerage in the Meiji era, and the beginning of the same process for Japanese and Korean elites (including some interbreeding of the royal families IIRC) during Japanese rule of Korea, although in the latter case WWII truncated the process and nationalist rulers of postwar Korea like to pretend assimilation never happened. ITTL, the same kind of process takes place early and long enough to be entirely successful. As a matter of fact, it is not really different from how the elites of the areas the European empires conquered during their original rise got assimilated in the dominant one. Centuries later, birthplace of your (noble) familiy hardly bears notice.

I share the opinion, this is why I proffered this solution as my best reasoned guess about what would likely happen, even if dynastic issues in general often baffle me.

To clarify a previous statement of mine, even if one's ancestors do not really matter for social status in any of the empires about ethnic heritage or area of origin, they do matter considerably as it concerns belonging to the nobility or being a commoner. On the other hand, given how much the HRE, the ERE, and NSE have been trade-minded, expansionist, militarist, and bureaucratic, a relative plenty of opportunities exist in these states for the talented, the dedicated, and the lucky to get ennobled or become the social equivalent of nobility in wealth and influence. I'd guess things in this regard are similar to Early Modern England and France or slightly/moderately better. So my tentative best assumption is these empires more or less cluster close to the threshold where the inevitable, coming pressure for transition into liberal democracy gets vented through gradual compromise reform, through violent (but not necessarily or even likely bloody or repressive) revolution, or a mix of the above. I guess this means the transition may take different forms in different empires much like it happened to OTL Western Europe. Things seems much more similar to their OTL equivalents for *Russia and *Ukraine, although as we said the LRC cannot really afford to be as dynsfunctional in its domestic policies like OTL PLC, so it doesn't.

Of course, to live in the capital also gives a would-be social climber or a member of the elite interested in expanding their own power, wealth, and influence a serious advantage. In this regard, I expect there is no realistic alternative to the capitals of the empires being Constantinople for the ERE, Kiev for the LRC, and Moscow (or possibly some other major city in the Volga bent; I am uncertain about the chances of Novgorod ITTL) for *Russia. For the NSE, London seems the obvious choice although I would not entirely rule out the possiblity of the court traveling between England and Scandinavia on a semi-regular basis in peacetime. The HRE is so vast, polycentric, and balanced in a demographic, economic, and strategic sense between its Northern European and Mediterranean cores that I expect a serious dualism between Rome and Aachen to exist, and my tentative best solution is the imperial court traveling between the two cities on a regular basis (say the Rhineland is the "summer capital" and Latium is the "winter capital"). Only in modern times when multiple capitals become far too much trouble, I expect the HRE would pick a logistically suitable city someplace in Switzerland as the compromise solution (obviously the reasons that led OTL EU to choose Brussels don't apply).  

Hmm, I broadly assume China and Japan-Korea have the potential to waver between relatively good, even friendly, relations, and tense, even hostile/antagonistic, ones, although never with the extreme, existential conflict we saw in our 20th century. To a degree, their strategic interests do step on each other's toes in East Asia, and since both nations are more trade-minded than OTL and non-isolationist (J-K even more so), they are regional trade rivals as well. Korea was China's vassal after all, and the Chinese would not take its permanent loss lightly; on the other hand, they got to accept Japan would not be a vassal of theirs (barring the Yuan conquest attempt), so after a while they should get to accept the new status quo. I am assuming at some point an ATL equivalent of the Ming collapse does take place, and Japan seizes the opportunity of a weakened China to merge with Korea, defeat or sidestep Chinese opposition, and consolidate a united Japorean empire. Such an empire should be strong enough to keep a resurgent post-Ming China at bay. However I'm assuming that even if a new Han dynasty replaces the Ming, Manchu takeover of China does not take place, so Manchuria and Mongolia never become part of the Chinese empire, and the former is up for grabs for intensive Japorean colonization. China would care for its loss, but nowhere as much as OTL (since it has no Han population). To a degree, this also depends whether Beijing is the capital. Since the Qing and their control of Manchuria and Mongolia are butterflied away, so does Beijing becoming the sole Chinese capital. It would keep sharing the role with Nanjing, or even being replaced by the latter altogether, if Chinese northern expansion is blocked and Japorean power would create a strategic threat. You don't put the capital of your empire close to the border with a powerful potential rival if you can avoid it and the status quo has not too much staying power. Beijing in this age has not that kind of traction.

As it concerns the contacts of China, Japan-Korea, and united India with the European empires, they are quite extensive at least with the three great seafaring/colonial empires, the HRE, the ERE, and the NSE. This means permanent diplomatic relations and flourishing trade which the European seafaring empires largely control up to Asian ports. Although the Japoreans and a lesser degree the Chinese do make an effort to be seafaring, trade-minded empires as well, they are still small fries and the Europeans do control the lion's share of global trade on favorable terms, in roughly similar amounts between the three empires. Cultural exchanges between the European and Asian empires are extensive and well-established as well. You should assume the elites of the two sides are thoroughly familiar with knowledge about each other and with goods of the other side, even if they don't personally own them. This has been the global pattern since the Europeans mastered reliable ocean-going technology and crushed the Muslim obstacle in the Late Middle Ages. Because of the reliability and overwhelming prevalence of the sea route for European-Asian trade, the land Silk Road has suffered a drastic decline, and Central Asia became a marginal backwater. The ERE does not really care, since they are in a good position to tap into the sea trade. Almost surely there are regular contacts between Russia, China, and J-K as well, since they share a land border, although at a significantly lesser degree because of the much greater logistical issues of the land route and since Russia is as usual mostly cut off from access to the warm seas. Much the same way, the LRC is essentially cut off from reliable free and secure access to oceanic trade and colonial expansion. The LRC and Russia however do get regular access to Asian goods and info through exchanges with the other European empires.

Much like OTL, Europe is unquestionably at the forefront of social, economic, scientific, and technological innovation, with about a century's advantage of accumulated progress on OTL, and is leaving the rest of the world in the dust. However the three main Asian civilizations (especially the Japoreans, but to some degree the Chinese and Indians as well) do show open-mindedness to be at least willing or even eager imitators and adapters, even if they yet can't really match European capability for innovation or challenge European global hegemony. The Amerindians and the Africans (at least in the places the Europeans may colonize safely) simply don't have a chance; colonization is the only realistic outcome for them. To an important degree, TTL has developed a remarkable level of globalization even before industrialization (of course in the terms feasible for premodern civilization). Climate and disease are keeping West and Central Africa mostly free from European colonial penetration, but for the same reason, the lack of transatlantic slave trade, and the limited importance of their goods for premodern global trade, they are mostly cut off from the rest of the world and its advances.  

I suppose you meant united India. Yes, and together with the HRE (the strongest of them all), the ERE, and the NSE, the Chinese, Japorean, united Indian empires make up the world's great powers. The LRC is constrained by geopolitics to a middle power role, short of fusing with Russia. The latter faces a similar situation, but also has the avenue of growing in status by properly developing the resources of Siberia and Central Asia, although that takes time and modern technology. Other great powers might arise if some European settler colony breaks away and keeps major regional/continental size. This is far from inevitable or even especially likely, however. TTL's circumstances make the eventual growth of the European empires into multi-continental Imperial Federations entirely feasible. Certainly such an occurrence would make the HRE head and shoulders superior to all its peers. I assume one middle power might arise in Indochina if one of its states pursues the modernization road timely and effectively like the Asian great powers. Otherwise the inevitable alternative for the region seems to become colonies or clients of the Europeans or their stronger Asian neighbors. Much the same way, the Mongol and Uzbek states, and the northern portion of Persia seem headed to be absorbed by one great-power neighbor or another. Permanent loss of Mesopotamia and western Iran doomed Persia to a severe and lasting dnowgrading of status, since the Khorasan-Afghanistan-Pakistan area has not the resources to compensate or resist its stronger neighbors. On the other hand, it questionable when and if Russia, China, the ERE, or India might make a serious bid to control the troublesome area, if ever. The remaining independent states of South India and Sri Lanka face an inevitable fork between absorption by the Indian empire or European colonization.