Personally, I think there shouldn't be any kind of competitive element here. I mean, stealth in Multiplayer is VERY tricky. How many of done that? The Ship and AC: Brotherhood are the only ones that come to mind. How would a sneaky character fight against a mage or a fighter, since their skills in stealth don't necessarily translate to real world skills unless you literally start to become invisible. So I don't think there should be any kind of PVP here, at least not directly.
Maybe create a miniature army kind of thing, recruit your own companions, outfit them, and then have them compete against another person's army.
But I am totally for Co-op, although that could create problems when using the menu interface. Would the game pause for both players? That would be unfair to the person forced to be paused. And would the game not pause at all while in the menu? That would be unfair for a character who needs something that wasn't hotkeyed.
Maybe create some dungeons or quests that can only be completed in Co-op. Like make it so that some switches must be activated simultaniously, and that'll lead to alternate routes or greater treasures, and everyone gets equal shares of treasure (in other words, someone takes everything from a chest, the next player won't find the chest to be empty, so everyone can get a copy of that extremely rare and powerful artifact).
Discuss, debate, pick apart, have at it. Pizza's in the oven.
"The error lies between the keyboard and the chair."-Dailchiban