How about a new zone or area where the zone is built like a normal PVE zone, same mobs, towns etc. The difference is when you enter the zone you have the choice of playing as a Mob/NPC OR as a normal player. There could be goals where the normal players try and hold towns or take objectives etc and people playing the NPCs or Mobs try and stop them, both solo objectives and grouped. The higher you are up the leader board on the NPC side the more difficult/larger type mob you get to play, like a Boss for example. When you log in you get to chose from what mobs are available. There would have to be rewards for both sides. Think Trails or Dungeons! Maybe in towns nobody can attack anyone or only in certain parts of towns.

It's just a rough sketch of an idea but it would be something different. Not exactly PVP since as a mob you have to play the mobs abilities like a skeever if you are low on the leader board lol.