French King Charles VIII Doesn't Invade Italy in 1494

  Brittany is unable to resist conquest without support and that is unlikely to occur before the French march in.  However if the trigger is elsewhere then it might preserve its independence for a while.  Or possibly find allies once it starts.

  The Hapsburgs and a lot of German states might feel threatened by such a large increase in French territories, especially since the northern part of the Burgundian inheritance was never part of historical France and it also makes the French king a very big player in the HRE. 

  The Pyrenees are a barrier but that goes for both sides.  Aragon has influence in Italy at the time as well so Spain could possibly provide a threat to southern or western coasts.

  England still has Calais at this point, and until the 1550's so has a potential point of entry as well as by sea.

  Also I note that the Wiki entry for Mary is that, as soon as her father Charles the Bold died she was forced, within a month of inheriting, to make major concessions to the assorted provinces of her state, see en.wikipedia.org/wiki/Mary_of_Burgundy#The_Great_Privilege .  If Charles VIII comes in and seeks to maintain control in a centralised fashion you are likely to see a lot of internal opposition.  Its possible you might get a parallel to his conquest of Naples and early retreat or of the Spanish problems with the Netherlands, but earlier and with a markedly larger state.  I could see France keeping the original province of Burgundy and possibly some elements on the Flanders region but you could see a greater Netherlands established a century or so earlier as an independent state or at least a very bloody war developing.

  A lot depends on the exact timing of events.  If the French move immediately after Charles the Bold's death in 1477 then England is still in the War of the Roses, although you might see the Yorkists under Edward IV staying in power and leading an English intervention.  Especially since Edward was the brother in law of Charles the Bold who has supplied funds to Edward when he had been threatened by Warwick, who was himself in alliance with the French.  See en.wikipedia.org/wiki/Wars_of_the_Roses#Edward_IV, especially the section about the clash between Edward IV and Warwick.

  I'm not saying the French won't win, or at least gain significant territories but its likely to require a lot of costly fighting if they try and force a marriage between Mary and the then very young Charles.  I would expect a long and bloody conflict, one side effect of which might be no Tudors in England, another possibly a unified Netherlands becoming an independent state prior to the reformation.  Which in itself could be drastically altered depending on events.

  Agree it would be logistically easier to hold the Burgundian inheritance than Naples in terms of logistics. 

  If reformists and devolvers of power win in France, possibly in part because of the Burgundian inheritance there are two potential problems with this:
a)  Their power is going to be a serious check on royal authority, so at some point this is likely to lead to some sort of struggle and possibly a civil war.

b)  If it doesn't wealthy merchants and lords often look with disfavour on centralised military spending so future wars of expansion could be difficult to maintain.  This was a major factor in the run up to civil war in England, that parliament had gained authority over spending and restricted what Charles I could do so he sought to rule without it.  Similarly with the Dutch revolt, while religion played a factor an equally important one was demands from Spain for taxes to fight wars that seemed to have no benefits for the Dutch merchants.

  In both cases Britain had the advantage of being an island nation and hence less vulnerable to invasion by neighbours, plus the British civil war(s) came at the tail end of the 30 Year's War when the rest of Europe was rather preoccupied and war weary.  France in either case is going to have the additional problem if social conflict leads to either civil war or paralysis of the government that this could prompt foreign intervention.  Although depending on the circumstances that is likely to make the immediately threatened area more accepting of royal authority.

  Another factor is that the reformation in some form or another is just around the corner so that could throw everything into the mix again.  Especially in large states such as the proposed greater France with a multitude of interests and cultures.  We're likely as OTL to see a lot of bloody conflict and chaos with the would be hegemony power facing the biggest challenge.