I have a multiplayer idea that would keep the game still very much single-player, but would also allow the guys that want to play with their buddies to do so:
I'll be using Oblivion as an example. And think about Gears of war 2's Horde mode, and Halo ODST's Firefight mode.
So, I go to the arena, and fight a little bit there. Then I get bored. So I go and either: Talk to an NPC and select a certain dialogue option, like "I want to find a partner!" then it would either host a lobby, or do a matchmaking system, and find another player, then we would both meet in the Arena. OR, instead of talking to an NPC, I would simply go through a door (load a new cell), and that would be like the matchmaking right there, when going through the door, it would matchmake, and all the players doing this would be put into the same arena. Well, not like ALL 400 or whatever, just like 6 per-group. But the door way wouldn't really work without some tweaking, because then there would be no lobbies, so friends really couldnt get together
unless they did it another way (and I imagine IF they do multiplayer, it will be lobby and not matchmaking). Or they could make it REALLY easy, and just make it so that when your in the bloodworks of the arena, you could just invite a friend (invite only) and they could join.
Then, once players are together, they would go through waves of enemies, like the Arena, but differently. Now, assuming a level 7 and a level 24 want to play together?.. Well, the only thing i can think of would be, the creatures would be the level of the higher player, but the lower player recieves less damage, and deals out more, making him a virtual level 24. or vice versa, make the creatures level 7, and the level 24 do less damage, and take more damage.. a virtual level 7.
I think that would be nice. Though I would still prefer a just a small multiplayer system. Basically free-roam (just like the actual game) but you can invite a friend. Idk. I just get bored with single player after my first to second play through.