If he returned to the House of Commons, then he'd probably be offered a (shadow) Cabinet position now. From there, if he could muster the support of his party, he could very conceivably end up Prime Minister one day. He is, after all, still a young man in political terms.
He's popular, certainly, and I don't doubt he has the faculties for the job; yet I still see it as fairly unlikely. He doesn't ingratiate himself with the party establishment: as someone above said, he's a maverick, not a careerist.