Slitherine

I do not propose a chat, but to be able to send a text message to each player, just like sending a request for peace, an alliance.
Like a peace, only one message per turn could be sent to a given player. There would be no spam because everyone would think twice because they can only send one message. Other players wouldn't know what the message is.
Now the sent message is visible to everyone and you cannot plot intrigue without revealing it.

Yes, that's something I'm thinking about too. The PBEM server is limited in what it can send to others players, so if something is done, it probably must be using the diplomacy interface.