Forming a duchy would be hard with the engine, and probably hard to balance. You can form a kindgom out of three duchies though, with the [something] DLC, at a pretty hefty cost. It isn’t organic, but I don’t know how that could happen.
- Declare an independence war, it’s right there on the interaction menu :P
- create/join a faction to oust him [some DLC required]
- get a claim, declare war
3.5) have a duchy in the HRE, if they’re still elective.
I agree, though, it’s not clear what you can do or how, or that, sometimes, it feels gamey. But I don’t have many ideas on how to improve on it, hopefully they do.