Something like Age of Empires, but with specialize cooperative play, ie one (or more) player manages and defends the base/economy, and other players command different armies that are part of the same team.
Maybe also throw in the ability to depose other members of the team (with various risks of trying to do so), so you never know who you can trust.
Theoretically this does already exist in AOE2, where in the custom lobbies players select the same player number/color and have total control of the same "player".
The backstabbing aspect would be difficult, with lobbies limited to 8 individuals, there would be a max of 4x paired "players" (e.g. pair1 & pair2 vs pair3 & pair4). If teams aren't locked like in FFA diplo matches it turns into a 3v1 (pair1, pair2, & pair3 vs pair4).
Unless the game could be modded to support lobbies > 12 or 16, the diplo aspect would be limited.
Maybe also throw in the ability to depose other members of the team (with various risks of trying to do so), so you never know who you can trust.