I would prefer it to be similar to like a gladiator arena battle. Your team fights a few waves a "trash" then a boss, then you get harder trash waves then another boss etc...but have the bosses randomly generate so you can't plan for the boss and just have to deal with what you have. I would also like it if you were stuck with the gear and spec you enter with. Only way to change is to wipe and start over, but risk having the waves and boss change upon re entering