1. Theoretically doable, but the "give away a county if above demesne limit" should cover thatI will certainly playtest the living soul out of this. I love the concept of the adjusted faction system.
Some possible&impossible suggestions:
-Faction provides the liege additional levies, but asks for lands conquered in holy wars to be distributed among its members.
-Faction provides the liege with additional levies, but expects him to press their claims on titles.
-Factions against a particularly powerful vassal within a realm. If realmsize is larger than say....30, and particular vassal (not liege) has 1/3rd, then other vassals can band together and either end up declaring war (if possible) or trying to make the liege cut the vassal down to size.
-Religious factions within multi-faith realms. Could be a demand from Cathar nobles on their liege to make the whole realm Cathar, for instance. In Orthodox/Miaphysite this can even be accompanied by demands to replace patriarchs. For examples nobles in Armenia can ask the Basileus to replace an Orthodox patriarch of Antioch with a Miaphysite one.
-Autonomous faction can demand that realm be split up (example: king of England&Scotland has two sons. They can ask the king to give Scotland to his 2nd son, or even a scottish noble).
2. Not really all that doable, and I certainly can't make the AI act on it properly
3. Not really doable; factions only target their liege or their liege's titles
4. Multi-faith realms are such a rare thing I'm not sure there'd be much point. Getting it to actually work properly would be near impossible