Idea: There could be a dungeon area, or perhaps simply a play mode, where players fight together against monsters, but with friendly fire taken into account. For example, if I'm chopping at a monster from close range, and you fire a Polaris blast at the monster, then the monster and I both get hurt. So we really need to coordinate our attacks!

Rationale: Elite players are all waiting around for Tier 4, or the Core, or a new Tier 3 boss, or whatever. Shadow Lairs have only partially filled this need for more challenging material, because they are so expensive. This is another idea for an intense challenge, that's genuinely different from yet another big boss or yet another trap-laden level. Friendly fire would be a big change in game play, but not as big as PvP was.

Details: To enter this friendly fire area/mode, you would have to have at least two people in your party (or three or four, even). Your party would be locked by default, so that random people couldn't join and beat you up for fun. Or maybe the party's members would be completely locked, as they are in Shadow Lairs.