Me too. I'm playing RTR Platinum which runs on RTW 1.5, (Whilst i await a more complete version of EB) and the diplomacy in 1.5 and BI is attrocious. The AI simply will not accept a ceasefire, not ever... Not even if they have only 1 city left, are on the other side of the map to you and their army consists of one unit of something levied... They still will not even THINK about accepting a ceasefire...

And protectorates? Oho... Don't even go there girlfriends. If you can get an AI faction to become your protectorate in 1.5 or BI then i'll eat my own hat, which in this case would be a white bandana with a black dragon on it.

The not accepting cities thing was purposely put there by CA i think... In previous versions people were giving their cities away and then retaking them and exterminating to cure overpopulation and squalor or unrest issues... Which wasn't how it is supposed to be played. But sometimes they will accept if you give them some cash first or as well as it.

All that said... I've been ignoring the diplomacy in this RTR game with 1.5... No point in even proposing anything... I have diplomats in a few cities but their purpose, along with the spy in the city, is to stop the AI from any dirty bribery antics... Sometimes they come to me and offer things like trade rights, which i never accept since i'm playing as Rome with some iron man rules, and ceasefires which i DO accept if the war is pointless and i've no intentions of expanding into their territories anytime soon.