This is for any Paradox game so some of you will have more experience in certain games may have the knowledge of what can and can't happen in them.
This is somewhat negative because unpredictability and huge diversions from history is one of Paradox Games best features.
CK2:
Powerful nation states (Charlemagne only): After the Carolingian Empire dissolves, the map becomes as disjointed and as crazy as a Hong Kong street map. The thing is, the Karlings fight each other over little scraps but nobody is able to replace them because they still team up against foreign threats.
Falkir: Unless you play as a Viking and make the old ways permanent, there will not be a Falkir. They just don't get strong enough on their own, but it's mostly because one of their holy sites is in the heart of the HRE. It isn't uncommon to see Norwegian kings rule England though.
Judean empire: Never seen it happen. Probably hard to do on your own too.
Republics: Honestly I've never seen them change the game all that much. They're annoying to deal with sometimes but that's it.
HoI series:
France puts up good defense: I only guess that Germany is so OP in the beginning is because AI can't replicate the genius of early campaigns.
Unholy Alliance:
One of the most fearful things that could ever happen (if you are allies or minor power) and it never happens and if it does, it expires immediately. I wish a better strategic game about WW2 would make this a hypothetical scenario.