Lol...so much whining and moaning...sigh.
Anyways I would personally love to see such a mod, keyword here being mod as it would be an optional expansion, and obviously(most likely) for PC only, so you console goers need not ***** and moan.
Borderlands managed to integrate a single/multiplayer RPG quite well, and for some of the aforementioned flaws, the skill system in borderlands scaled to the highest ranked individual, so yes the enemies were hard as shit for a noob, but they leveled up very fast as a result being that experience for killing enemies was shared across all players not just those who did the killing.
Bethesda themselves said there was not going to be multiplayer so this would be a player made mod. It will(would) also take some time to develop so I doubt it would "ruin" the experience, most of us will be well through the game by the time it would come out.
It would be no where near MW3(and comparable shooters)...and its...well interesting gameplay (which goes back to the mod most likely being PC only).
I think what borderlands did was a great achievement in gaming honestly...it had a very unique approach being that the "Host" could be whomever, and it would load their world for the other players, competing missions and quests were saved for the host game only, if players went back into their single player games, their story did not progress leaving little issues with it messing with your SP game. The game also scaled progressively in difficulty, being the more players that joined and their collective levels would make the game more difficult (more HP for enemies, because more damage could be dealt).
This is really an amazing and immersive(sp?) game and I can see bandwidth being an issue for online play, and the task in general being quite hard to accomplish...but I am hopeful that in coming months or years after we have all completed or tired of the game perhaps a co-op mod will come along and revive gameplay for some of us.
-Rd