I know there have been talks of 5v5 DM, and Capture, but until then, maybe the weekend events (or permanent) could be a little more challenging that EA could easily change.
COMBINE enemy forces. They did it with the banshee challenge that we failed. Maybe have a wave of cerebus, a wave of reapers, and a wave of geth. Or a wave of some geth, and some reapers. Or a wave with all 3. Maybe you could have a crap-load of enemies on a level, and they fight each other, and against use, in a last man standing winner take all event.
Yes, I know it doesn't make sense to have the geth fighting alongside with the reapers, but does it really matter? The first time I played after losing the banshee challenge, I played against cerebus, and on wave 5ish, I heard a bansee scream. It scared the crap out of me. Wasn't expecting it, but I loved it! Bring on multiple enemies!!!