I saw some posters above mention Juan Cena and my guess is that this gimmick is reserved for house shows. That way those shows that have advertised John Cena can still satisfy the fans while still keeping the Cena fired story intact. John himself being on Raw has been the big story and there's been no reason to bring Juan to television.
That aside, I think there are three main ways this could go:
1. Cena is rehired, nexus is intact, and cena continues to feud with barrett. While this may make the most logical sense, I think it is actually the least likely outcome because it feels too much like nothing has changed from where we were when this whole nexus/cena feud began.
2. Barrett refuses and is kicked out; when the dust settles Barrett stays heel and nexus become face/tweener. This would probably be the strangest option. Raw doesn't need a face faction because to be honest, a face faction is pretty uninteresting. And if Barrett stays heel there's no one else besides Cena for him to really feud with.
3. Barrett refuses and is kicked out; when the dust settles Nexus remains heel and Barrett slowly moves towards tweener, maybe even slightly face. This is the one I think will actually happen...hopefully gradually, but because it has the most potential. Especially if at some point Barrett gets in a bind of some sort...maybe against former nexus members, and actually needs to enlist Cena's aid to help him out. Go the whole former enemies/reluctant partners with a common goal route. That could be interesting and could leave Barrett in more of a face position which, with the current raw roster, would give him more possibilities once this storyline is all over.