Yes, yes, I can hear even hear the upcoming complaints in my ear while writing this stuff, but please read my text first....
So, Player vs. Player, one of the BAD MMO modes....yes, indeed. IMO
a PvP arena mode would be awesome in D:OS! I mean, actually there is already almost everything in the game for a mode like that, you would only have to tweak some things, make a proper player lobby (which is reasonalbe for co-op anyway) and make one or a few small maps/arenas.
In fact, the engine supports 6 player characters. There could be mods later with content for 6 players at the maximum. So it should be possible to let these players also fight against each other, I mean honestly why not? That could be real fun. Sure, perhaps it could be integrated later by fan-mods but why passing on something in the first place which could enhance the game and attract even more people to buy the game?

1v1, 2v2 or 3v3 �� Could be great fun with friends and wouldn't be that much effort I guess. I think more game modes are always a good marketing instrument for reaching a greater audience. And the game is already co-op based so why not using the possiblity to invent a small PvP mode, just an arena in which two teams can fight a turn-based battle?
Co-op and PvP are like siblings. One is WITH another player the other is AGAINST another player. I don't get the point why there shouldn't be both available in D:OS.....