in this game mode you have limited ammuniton.
pretty simple concept, but makes for an interesting fight i think, people would not be able to constantly spam out shots and had to think about when and where to shoot
there should also be a limited number of charges for abilities, you know to avoid everyone just using missiles to kill :D
if the format is tdm, then you should respawn with the ammo you had left or with maybe 20% less than you had when you started the match, to avoid people just feeding to regain ammo
could also work with team elimination maybe....