Was there already a Germany at that time? From what I recall, Austria was an empire, but 'Germany' was more of a concept like 'the Free West': A bunch of larger and smaller kingdoms, electorates, palatinates and even some prince-bishopricks, united only by the fact that they all speak some form of German and loosely have a similar culture.
I think the main What If would be if one of the bigger players like Prussia or Saxony would get a Bourbon dynasty, could it go ahead and unite itself and its surrounding kingdoms into a German superstars much like the French kings did 300 years earlier.
However if the idea is what would happen if the Greater Austrian empire would become a Bourbon dynasty, all bets are off the table.