Best case senario Kameo's are much more than just assists, with a player having much more control over a kameo character than just them popping out and doing a special move. Like they have strings, drastically alter combo paths, and different pairings have huge effects on how you play a roster character.
Buuuuut, as I say this I cannot help but compare them to variations, as others have mentioned. which imo worst case scenario is kameo's are variations 2.0. So its a weird balance for NRS to find where Kameo's NEED to be a intricate/more complex part of gameplay (cuz how much will it suck to have your fave characters be stuck as merely a couple of their special moves) but roster characters must be fully fleshed out and complete on their own to not limit a players ability/creativity (ala variations). Hope that makes sense. I do have faith in NRS tho.
One thing tho is I think NRS will absolutely KILL IT with the during fight banter between whatever roster character and kameo you pick. Especially if they're normally rivals. Like there is going to be a lot of gold in that regard.