In my opinion the diplomacy could be improved a little. For example diplomatic relations only describe how they feel about you but you can't set how you feel about them.Relations like that are always bilateral. If the Yor have warm relations with the Dregin, the Dregin will have warm relations with the Yor. The same applies to you.Paying one civ to attack or make peace with a second civ does not affect the second civ's relations with you, as they have no way of knowing who did it.
Then relations could be more realistic. Two parties in a relationship don't always have the same attitude towards the other. They'll probably still be similar most of the time, but it could create some interesting diplomatic dilemmas if they could get out of sync. Stuff like historical grudges (Yor/Iconian, Altarian/Drath, etc.) might have one race disliking the other more.
The game sometimes says "your agents have reported..." to inform you of major events, so even if it wasn't done openly it could still be well known (usually there's an exchange of some kind taking place). And there's no reason a payoff to stop a war like this couldn't be made in open diplomatic negotiations. There's rarely a reason for a peace-broker to hide. I can't think of any real life peace-brokers who preferred to remain anonymous...
It would be more interesting if you had the option of doing this in the open or in secret, but I'm not sure if there would be much point of differentiating.
And how cool would it be to have diplomatic negotiations with multiple races at the same time? Either yourself and two other races, or maybe even more than that, kinda like a UP session. "Realistically" that's probably how it would work.
Comeon Stardock, that'd be another fun feature to add in. I'd jump at the chance to design a system like that, and the AI logic that goes with it.