This is a really cool idea. It reminds me of some WoW battlegrounds, except those didn't have full loot. However, it was a great opportunity for PvE people to take part in PvP.
Couple of questions/clarifications:
-I assume if you get killed, you can't re-enter?
-If all the other teams are killed, but there are only a couple on your team left and you aren't strong enough to beat the final boss, does everyone just die and lose all their gear? I'm not saying this is a bad thing, it's actually an interesting outcome.
-If your team wins, do you just loot whatever you can carry and leave? Which means, unless you have floating discs or huge containers, you may have to choose carefully what you loot. If a bunch of your team died, you may have to choose between CRing them and looting other teams. Again, this isn't a complaint, it's a neat scenario.