If they do go back to WWE, I don't see them being as big as they were in TNA or the early 2000s in WWE. I could see them being brought in to work the tag team division, but don't see them doing anything spectacular, since they won't have the Hardyz and E&C to perform against. Those two tag teams really carried them in those TLC and Cage matches.
I know some may say, "Ray is over as a main eventer". I do agree, he is over to a very small demographic of the wrestling fan base. Remember, the majority of the causal WWE fan base, doesn't watch TNA. Ratings, PPV buys, and merchandise sales reflect that.
If they go back to WWE, I just don't see them throwing Ray in a big spot over the younger guys they're pushing to the moon. He may work with them, but only to put them over.