I can't see the nWo coming back, because it isn't an original creation of WWE, and you know how they've been about original creations in the last few years - FCW names, anyone?
I honestly believe that the only reason the nWo appeared back in 2002 was because Hogan, Nash and Hall all wanted to return to WWE, the opportunity was too good to pass up and most importantly, because they needed a way to bring in Hulk Hogan as a heel and they weren't going to turn The Rock at that stage. It kept going for a while afterwards because there was some initial interest in it and WWE was quite happy with what it was bringing in via merchandise sales.
If anything, I think the most likely stable of the past to make a return is the Four Horsemen, since Ric Flair looks set to return shortly in some kind of managerial role. There has been talk about pairing him with Dolph Ziggler. When I think about it, I could see Flair resurrecting the Horsemen (like Fortune, but the proper name, licensing and entrance music) with Dolph Ziggler and 3 other superstars that WWE wants to push, who need a legendary mouthpiece and the prestige of being a member of the Four Horsemen to their names.