I think that it'll be Cena.
I don't know why, but I think they will avoid this and go for someone fresh (or, if they are really lame, they will give it to the Rock in a part time stint).
They seem to be in love with Ryback and I really think it would be a direction they go in to have him win the belt sometime next year. This year is too early, but, they are clearly giving him the push needed go get in a serious contender spot instead of just randomly handing it to him.
If, somehow, Punk vs Austin happens at WM, then I think Punk is most likely going to hold the belt until after WM, because he wont drop it to Austin, who would more than likely just show up for WM and then leave after.
Would be sick as balls if Austin came back full time but it ain't gonna happen. He is in no shape or form to do so....