Yep. It's been brought up many times. I'd quite like an endless mode where you fight til your fireteam has all died with increasingly tougher mobs. Another version could be a 15 minute timed event with revives - kill as many mobs as possible. Could be weekly leaderboard of the group with the best scores. Or simply make an epic public event mode where you select it from orbit and there's a full team of you doing a bigger version. Multiple walkers or warsats to defend etc. All with 6 player matchmaking. :)