Sounds like your classic "side game" that you see in many multiplayer games i.e zombie mode, survival, etc. I like the idea but would like to add power-ups upon defeating mobs like "Full Cure" or "Attack Boost" that will add dimension to the game. Large arenas with multiple levels (similar to T2) with random enemy spawn points will keep the party on their toes. Combined with power ups dropping from random enemies and places on the map you as a player and as party need to decide whether to risk running across the map for a "Full Cure" but run into a possible ambush or try to stick it out in your area with dwindling MP/HP/TP.

Either way this would be an awesome addition to the game.