You need this, even recycle old pvp maps and have enemies spawn on them. Skulls as difficulty increases, weekly challenges, and gear related to waves reached ... Do it.
It's a good way to practice with clans, buy time as the game needs time to improve throughout DLC's, and overall, just a fun game mode with friends.
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Sad part is if they put this in game, they would only need 2-3 PvP maps to accomplish it and it would quell alot of the anger people have. Skyshock would be perfect. But the masses would expect drops and loot from it instead of just the fun of doing it.