It would be a complete outsider - people like Dwayne Johnson, or Tom Hanks. Maybe even a primary challenge from Arnold Schwarzenegger.
The right wing has singled out most of the people on this poll as tangible political threats and have engineered attacks against them - and frankly, in the Trump era, those attacks work to shut down sanity and rationale. Under normal political circumstances I would have voted for a ticket that included Sen. Warren, but her odds are not good owing in large part to how much static the right-wing has made about her being "Pocahontas," accusations which frankly, her rebuttals have been less than concrete against. It's another wave of Birtherism. Even if she did produce a DNA analysis, the right would try to argue the results are bogus as they did with President Obama's birth certificate. Bernie will not (or should not) run again either, I don't think enough voters take him seriously not just for his Democratic-Socialist platform (which will pick up steam in coming years anyway) but because he doesn't strike the average person as a strong leader, he is seen as a cranky old man that is up in his post-retirement years.
While it would make sense for the policy and the stability to elect someone like Michelle Obama I don't fancy her chances either. The US has no interest in dynasties. Al Gore came real close, but his competition was also the son of the then-newest former president, GHWB. I don't think given the choice Americans really want that at all. For that and other aforementioned reasons Clinton, Gore, Biden, and anyone else that was a direct insider of a prior administration need not apply, the "Deep State" conspiracy would ruin their message. That's why I think it is going to be another Washington outsider, to compete with a "Washington outsider."
It would be really hard to accuse The Rock of being a Deep State operative, or to find something wrong with him when Americans from all sides have found reasons to like and follow the man's career, from his fake wrestling "Candy Ass Jabroni," People's Elbow days to his work in films ranging from Disney characters to action movie heroes that just jump out of hospital beds, arm-flex their casts off, and go beat the crap out of bad people. And that's how he got rich, not by making curious real estate deals with Russian oligarchs or working his way through state government and enriching himself dubiously along the way. His work with Disney especially will work for him. The House of Mouse doesn't tolerate anyone who doesn't exude a wholesome image, and I doubt there are any James-Gunn-like tweets hanging out in his closet that are going to torpedo him with static in a campaign. Johnson has already made it clear he's considering a run and it honestly seems like one of the least absurd choices out there. The most anyone could try to attack him for is being an actor, and thus untrustworthy - but given not just the fact that Reagan is a former president, but the fact that Trump is a demonstrably pathological liar himself, I think this would be a really desperate vector to hit him on. I don't know. I'd like to hear what others think, as I can't think what oppo is really going to injure him if he did try to run.