Getting rid of all city states will kind of remove the world congress gameplay and the Diplomatic Victory as well,
Hmm. When I go for the Diplomatic Victory I might try to lower the number of players, but increase the City-States. Essentially forcing me to interact with them more and learning how to handle them properly.
You totally hit the nail on the head about my issues with them as well! As I was going on my conquest, I kept losing track of who was at war with me and who wasn't. Hell at one point I attacked one of them and a nearby one just started helping me as I didn't even realize they had cleared a quest from them.