It's pretty much a lock now that Kevin Nash will be returning to WWE under the "Diesel" gimmick. It's possible that he'll debut at WrestleMania, although it's not a guarantee.
The likely return date for Nash is the RAW the night after WrestleMania in Seattle. Wouldn't be the best move since he would take away from Goldberg's debut, and vice-versa, but that's the plan.
It's also likely that he will be involved with Shawn Michaels at some point upon returning.