The only affordable way I could see doing something like and MMO, would be make it similar to Guild Wars. Have one main server (and actually that could even be forum based since game key's are connected to the forum I believe) were you buy your equipment etc. and then have people host there own servers for there own group/party/clan.
This is sort of what cRPG has done, but with just the regular game mode. What would be interesting is to make something along these lines with the SP part of the game, and then have the cRPG style of experience and equipment.
The main problem would be that the world would have to be pretty big, and probably not have an over world map like they do now in SP. A possible solution would be to have a map system, were the party leader had a map that would cause the party to travel to different areas. Example of how an area would work, say the area of Praven, it would be bigger then just the city, that way you could have battles outside of praven, and hunt down bandits etc.
The main problem would be the AI, with merchants and neutral NPC's.
You could actually even make it have a lose story line if you wanted, and perhaps even a story line for each faction.
I do not see how an MMO would be possible with the current technology, although maybe 10-15 years from now they will have the technology to host a server for warband that can hold 1000+ people. (on that note, the reason why warband would not work MMO style would be due to lag, lag is not AS big of a deal in a game like WoW, since it is not about reaction time, but with warband lag is a major issue).
Hmm, this has just given me an idea for a project...