Okay, so this AHC is pretty simple: Create a unified German state at the worst possible time using the worst possible nation to do the uniting in the worst possible way both for Germany and the rest of the world, though, if priorities conflict, make it worse for Germany first.
The unification should ideally be as destructive and violent as possible, the unified state should be as authoritarian and unstable as possible (if you have to choose one or the other, go with the authoritarianism, though the two often go hand in hand), and international relations between Germany and everyone else should be... difficult at best.
Mess shit up with the German unification, basically.