But what if diplo games had their own tab under multiplayer? Or would that be unnecessary? What do you all (except for JSO) think?
I don't think it's good, or necessary.
There are lots of steps in-between a pure strategy game and a full-on Diplomacy game. Forcing people to choose "which kind" their game is... is a bit of a challenge, and a totally unnecessary complication.
I'd like to keep the ground clear to start any kind of game you want. Maybe you want a game which is pure strategy for 10 turns, and then 5 rounds of diplomacy, and then back to pure strategy? Why not! (It is, of course, up to YOU to be as clear as possible in the game's description.)