Well, the game is pretty solid on its own. NWN is a great game as well and while I enjoyed the MP aspect at times, it never really made the game for me. DA excels at storytelling and roleplaying. The combat system is solid and fun but it is the character interaction that makes the game. Trying to work MP into the SP game would be tragic I think. I'm not saying that the engine itself would not be fun to play with friends.. but the SP campaign would, in my opinion, stink. Anyhow - my call is that with the amount of backstory/history/detail they put into this game we will be seeing a DA MMO in the near future. To me it just feels like they were thinking about one from the start. We'll see.
As an example of another great engine that was modded to heck into a great, great game I offer Oblivion. No MP component and a so-so (read:weak) single player campaign/engine that was turned into a powerhouse of fun through the likes of MMM and OOO, among others. That game STILL did not have the immersive RP experience that DA gives us but it was still a great game in its own right. The open world is one aspect I would rather have seen in DA. The smaller "zone"-like areas break immersion a bit for me but not too badly.
I am excited to see what the world does with DA... although I have a hard time imagining that anything like the numerous, numerous, numerous dialogue scenes with full voice acting (except, eerily, for your avatar) can be done through a user mod.