Currently, only outposts can be claimed. I could claim a city from an opponent much weaker than me to try to "demonstrate my power" and if he refused it would generate complaints for a casus belli. I don't see much sense in being able to claim only outposts, and claiming cities would be a good way to justify a war.

Do you think it would be a good idea?

No? It would make war pointless. If you could get everything war gives from just claim with expansionists.

You can claim anything if you have demanded your grievance on it. Even cities. Even their capital city. Just make sure you demand it before you go to war.

It can, just have more influence/religion over it and you can demand it.