5 most likely people to become the 47th POTUS?

[Mister Mets]

Interesting question.

Nikki Haley: Well-positioned to run following a Democratic President.
Ron DeSantis: Popular Trump-friendly Governor of large swing state. He can run for a few cycles. He might pull the old trick of losing a nomination that isn't worthwhile, and getting nominated the next time around as the also-ran. He would also benefit from any Republican backlash to perceived identity politics.
Tammy Duckworth: Plausible Vice President for Biden. Could run in her own right in 2024.
Cedric Richmond: On paper, he seems like a plausible Warren running mate (younger black man from Louisiana who is part of the House leadership and an ally of Biden.)
Mike Pence: Even if Trump loses, he'll be a top-tier contender in 2024, when we may have an open election or someone with questionable electoral prospects running for a second term.

[Mister Mets]

One thought experiment is the odds the 47th President is a member of a particular body.

If Trump is unable to finish his term, Pence is pretty likely to be the next President (or he'll be swapped out Spiro Agnew style, so another Republican will get it.) This means the 47th President would be whoever follows the Vice President. This includes the possibility of Trump winning reelection but still leaving office early.
Odds are good it'll be a Democrat, as voters will eventually get tired of Republicans in the White House. (I'm guessing odds of this scenario are 8%)

There remains a small chance of three Republicans in a row. (2%)

If Trump loses reelection, the 46th President is almost certainly a Democrat. The next question is whether their successor is a Democrat, which is likelier than usual given the age of the field. (25%)

But it's also possible that whoever defeats Trump will be followed by a Republican, someone who defeats them in 2024, defeats their successor in an open election in 2024 or defeats their successor in 2028. (20%)

If Trump stays in office until January 2029, there are a few possibilities.
A Republican wins and is followed by a Republican, which can also happen with 46's resignation/ premature departure. (6%)
A Republican wins and is followed by a Democrat. (12%)
A Democrat wins and is followed by a Republican. (17%)
A Democrat wins and is followed by a Democrat. (10%)

I'm pulling the numbers out of my ass, but I've got the odds the 47th President will be a Democrat at just under 50 percent.

I had more Republicans in my top five, but that's more because the Vice President and two rising stars in their 40s can dominate the rest of the field. The 47th POTUS might not win the presidential election under 2032, and that could very well be a Democrat who is currently completely unknown.