If you're playing with someone who steals your loot, they don't understand the game.
Or you dont understand what competitive questing means.
Know what? I will be EXTREMELY curious to understand how they are planning to balance challenges ahead in a way that works for players who are cooperating nicely and for those who are going to "split the team in two and go their own way".
I think there are 3 possible scenarios.
1. Players will ignore any competitive tasks.
2. I tell my coop partner: Hey dude turn back I have a quest to kill you, so dont make a problem. Then mate does as asked.
3. Players make an evil blood so they stop playing in a while.
Anyway, dont see any "fun" scenario available. C operative play is about, ehm cooperation and success together against odd. Asking for a complicated system of loot which negates sick designer idea that player should fuck out each other cooperatively doesnt have a sense.
