I can't see it happening, but I would LOVE to see Lesnar Vs Bryan!
It could be like a modern day Lesnar/Angle match, with lots of mat/amateur/Japanese style wrestling.
However, I think it will be either Punk or Orton.
Having fought both John Cena and Triple H, Punk and Orton are arguably the next biggest stars in the company so it would be one of them.
Failing that, it'll probably be Sheamus, promoted as two big guys, one the past and one the future of the WWE.