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.
If it's June 2016, and Mitt Romney's not the Republican party's nominee for President, he couldn't expect a chance for that to happen in the future.
He might have incentives to be a Biden or Cheney type, a conventionally qualified Veep who can take over in an emergency, add gravitas to a ticket and advise a President. I'm not sure he'd take that deal, but it seems plausible.
Now that there is a Mitt Romney 2016 megathread, and for the record, I do not believe for a moment that Mitt will run for President in 2016, or ever again, but that aide, for the purposes of sheer speculation, let us say that Mitt did run in 2016, knowing what we know now, who would be the best running mate for Mitt?
Give your reasons for your choice or choices if you like.
He got along with Paul Ryan, so there would be some advantages to doubling down on that. There are plenty of face-saving outs available to avoid that. IE- Paul Ryan believes he ca do more in congress.
Top Republicans elected in 2010 would be more qualified. A reelected Susanna Martinez would be a great choice for any Republican, and fit a theme of Republican governance. A reelected Nikki Haley wouldn't seem like a token pick.
And there may be new Republicans elected in 2014. Cory Gardner would be valuable in a key swing state. A Senator Joni Ernst has a lot of checkmarks (Young, Swing-State, Geographic diversity, Military background, Woman, Legislator, Early friend of Romney's.)