When there are still regions left, it is still possible to claim a new city. Takeover, RTO, revolt. You used to be able to buy one, not sure if that is removed now or not.
I think though, that if a realm does not have a bank available due to not having a city, that the option to build one should become available. Same goes for troops, only allow people to recruit them directly from the RCs instead. Second one makes tons of sense. Since the recruits are no longer being sent to the capital after they're trained, they are stuck at the RC they were trained at, no?