Assume that, following the failure of the invasion in Russia, a Sixth Coalition against Napoleon still forms, with Prussia and Austria among them. However Napoleon somehow manages to win anyway, say because more of the Grand Armee ends up surviving the campaign into Russia, nevermind how exactly but assume that his victory in the war is convincing, and he ends up occupying large swathes of the Habsburg Empire and of Prussia, the remainder of their armies unable to challenge them and the Russian armies are forced to pull back unable to free their allies' lands. Possibly Vienna and Berlin are also under occupation. At this point the defeated powers sue for peace. Napoleon is in a position to impose a very harsh peace. But how harsh would it be?
Austria has declared war on France 5 times within the preceding 20 years, and ITTL was defeated all 5 of those times. It's probably clear to Napoleon that they can't be trusted at this point, and he may be tempted to completely dismantle the Habsburg empire to avoid any future conflict coming from that region, but even though personality-wise it would suit Napoleon to do that practically it has a lot of problems: first of all, that kind of harsh peace imposed on a defeated power is almost unheard of in contemporary European politics and may alarm every other state including his own allies; secondly, his wife is a Habsburg princess. So the Austrian will have to give some concessions, probably Galicia goes to Warsaw, and the Illyrian Provinces expanded. But what else? And how does he make sure the Austrians don't betray him again?
Prussia is a slightly different matter. I think it's much likelier that this country goes to the chop. There was fear this might happen already after the Prussian failures in the Fourth Coalition, and while Napoleon satisfied himself with "only" reducing Prussia's size and stationing garrisons on its territory, in part because he didn't want to humiliate the Czar who was Prussia's ally, he felt personally betrayed by the Prussians and their failures in battle led him to totally look down upon them. If they're defeated again in the Sixth Coalition, Napoleon might not be so lenient. If he does decide to completely dismantle Prussia, what happens to its territories? Silesia - to Warsaw maybe, or perhaps to the Habsburgs to sweeten whatever deal he imposes on them? East Prussia - also to Warsaw? Is carved into a new state, and if so rules by whom? Perhaps given to the Russians to entice them to sign a favorable peace? And what about Brandenburg proper, would the Hohenzollerns still keep it as a rump state or would Napoleon install someone else (who?) on its throne?
Austria has declared war on France 5 times within the preceding 20 years, and ITTL was defeated all 5 of those times. It's probably clear to Napoleon that they can't be trusted at this point, and he may be tempted to completely dismantle the Habsburg empire to avoid any future conflict coming from that region, but even though personality-wise it would suit Napoleon to do that practically it has a lot of problems: first of all, that kind of harsh peace imposed on a defeated power is almost unheard of in contemporary European politics and may alarm every other state including his own allies; secondly, his wife is a Habsburg princess. So the Austrian will have to give some concessions, probably Galicia goes to Warsaw, and the Illyrian Provinces expanded. But what else? And how does he make sure the Austrians don't betray him again?
Prussia is a slightly different matter. I think it's much likelier that this country goes to the chop. There was fear this might happen already after the Prussian failures in the Fourth Coalition, and while Napoleon satisfied himself with "only" reducing Prussia's size and stationing garrisons on its territory, in part because he didn't want to humiliate the Czar who was Prussia's ally, he felt personally betrayed by the Prussians and their failures in battle led him to totally look down upon them. If they're defeated again in the Sixth Coalition, Napoleon might not be so lenient. If he does decide to completely dismantle Prussia, what happens to its territories? Silesia - to Warsaw maybe, or perhaps to the Habsburgs to sweeten whatever deal he imposes on them? East Prussia - also to Warsaw? Is carved into a new state, and if so rules by whom? Perhaps given to the Russians to entice them to sign a favorable peace? And what about Brandenburg proper, would the Hohenzollerns still keep it as a rump state or would Napoleon install someone else (who?) on its throne?