If he comes back to WWE, then it'll be in a non-wrestling capacity. He may get a Rumble appearance, but he won't be wrestling any matches beyond that. Vince/WWE refused to let Flair wrestle since he returned (I think it was something to do with the network not wanting Flair bare-clothed on screen), and I think a similar stance will be taken with Hogan should he return.
As far as how likely Hogan is to jump back to WWE, then it's really anybody's guess at this point. There's reports saying that TNA could do away with him, but on the other hand there's reports that TNA want to hang on t him. It's pretty much 50-50 to be honest.