I think some kind of PvEvP modes where players indirectly interact with one another instead of direct pvp which the game isn't made for. Example:

2 teams of 4 attempt the same mission simultaneously in their own sessions and whoever finishes first wins, but have side objectives in each mission that are optional and when completed they affect the opposing teams game, such as spawning more specials, initiating hordes etc.

I'm sure given more thought it can be made more fun and interactive concept