It wouldn't make sense for Romney to wait until 2020. It's a few months before the 2016 primary is officially underway, and Romney's considered such a strong contender because of a series of lucky breaks unlikely to repeat in a different environment.
I don't think he wants to be Governor again.
He could be a strong Veep. The public and media know him, he's unlikely to get caught in a major scandal, and he could balance tickets for several top Republicans. His connections could be useful in a presidential administration, and his talents are suited for a Veep with a portfolio (IE- put in charge of several major task forces).
I don't believe Mitt Romney wants to be VP. Being openly on McCain's short list in 2008 was one thing; it was a way to advance his standing for the future. Now, however, he's a guy who's been a nominee, and a losing one who's taken a lot of criticism. Being a VP candidate at this stage of the game is something that will ensure that Romney will NEVER be President.
Being second fiddle to any of the lame crowd of Republicans running could never benefit Romney. And he's not the kind of guy that would be a good VP. He's not a "hatchet man" and how would it look when Mitt Romney, who has stood on his own in business at an EXTREMELY SUCCESSFUL level is reduced to being chairman of the Competitiveness Council or some lame task force? It would look like a highly successful man lowered himself for limelight.