Let me put it in another way. When recounting my exploits in the recently released GTA IV, I can have different types of amazing experiences from the offline mode versus the online multiplayer mode. I think that MMOs can harness this as well.
The recent purchase of Diablo3.com by Blizzard was perfect in its timing, as it got me thinking about what that game could be like. I really think that if Blizzard was going to make Diablo 3 at this point, it wouldn't be anything like the last two games. Diablo is dead, Baal is dead -- so what else can be done at this point?
A new world with new characters could be one thing. Maybe even a new storyline involving Diablo himself would be worth exploring. The big difference could be in the way the game is designed: as a single-player MMO.
It's simply, really. If you've ever played the first two Diablo games then you remember how much fun the single-player experience was. Just keep the singleplayer side of the game while throwing out the multiplayer aspect for more of an MMO experience. Blizzard could use an upgraded version of their Battle.net service to have players login to the play the single-player game (as to avoid cheating and such) and then if players wanted to they could jump into the MMO version of the game at one point or another. Of course, in Age of Conan you have to wait until you're level 20 to get into the full-blown MMO aspects of the game. I suppose a similar restriction would have to be put into place for Diablo 3.
All it really comes down to letting people choose the kind of experience they want from an RPG. Nobody has really done a full-on single-player RPG that dovetails into a complete persistent online RPG. There are all sorts of chances to blend together different gameplay experiences with a single-player MMO. You can have the traditional solo experience, play with a friend co-op style or even play the entire game like a big story-focused instance with five or more friends. Just imagine the kind of water cooler-style discussions you'd be able to have with friends about all your shared exploits. There are some very cool possibilities here.
Allowing people to play in the way they want with the people they want is what's most important about this idea, that's the single-player MMO at its core.
These are the weekly, random and sometimes paradoxical ramblings of Kyle Horner. He blathers on about MMOs every Saturday for both his enjoyment and hopefully yours as well.