IMHO the problem remains Emperor Napoleon, sure he was in retreat on many fronts and had suffered a heavy defeat in the Battle of Nations (Leipzig), but character-wise this would have meant given up more than he had. I'm sure some French now think, why didn't he, well IMHO his OTL last 100 days might be telling here. And then for the elephant in the room here, I'm pretty sure Metternich would have ended up being cursed by a number of Germans once the age of Nationalism hits (some might in hindsight even deem him a traitor, in that period that IMHO could be real possibility), for allowing major German towns like Cologne, Mainz and Trier, all former Prince Electorates of the HRE, and an Imperial coronation site of the HRE like Aachen in French hands. IMHO that's much more dangerous than OTL French (owned) Alsace ever was.
Then there also is the issue of the fate of Flemish and other Dutch (as in language group) speaking groups of modern day Belgium had they remained a part of the French Empire ITTL. OTOH groups like the Germans and the Flemish, which remain under French control ITTL might end up causing problems for France in the future. That might spark a new conflict between the German states united under Austro-Prussian leadership to support German insurgences in TTL France and aiding them for the 'liberation' of Mainz, Cologne (Köln), Trier and Aachen (Aix-la-chapelle).
IMHO such a future conflict would be a real possibility and given what's contested ITTL the stakes will be higher than IOTL. The Netherlands will be a bit of wild card, that might depend whether they align with France or Prussia, and in case of the latter unrest TTL Dutch speaking areas of TTL France might influence that even further.