In terms of likelihood, well I made this list a while back
Here's my guesses (note not my preferences, but who I see as the most likely)
1. Andy Beshear (easier to pick him if he wins in 2023, but his chances still are decent, even if he loses): Governor from a red state who has handled coronavirus well. His moderate image could help Harris reach out to moderates, and while he won't put KY in play, Southern whites in places like NC/GA/FL might be more likely to see "one of their own" on the Democratic ticket
2. Pete Buttigieg: Young and charismatic former mayor from the Midwest. He will likely get a foreign policy spot in the Biden adminstration, and if he does he would help the Harris ticket on foreign policy. Also has a moderate profile, similar to Beshear, which allows him to reach out to moderate voters, however, would also be a historic pick as the first LGBTQ+ person on the ticket
3. Andrew Cuomo: NY Governor who has received praise on his handling of the coronavirus and has experience in Washington-style politics as he served as HUD Secretary under Clinton. Generic mainstream Democrat who's acceptable to all wings of the party
4. Tammy Baldwin: WI Senator, who has a record of winning in the key state. Fairly liberal profile, so an olive branch to progressives, who might not be completely on board with Harris, if she runs a very cautious/moderate campaign, however still has a record of winning by fairly large margins in WI. Also would be a historic pick as the first LGBTQ+ person on the ticket.
5. Stacey Abrams (this really requires her to win GA-GOV, but I think she will): Young, charismatic and an olive branch to progressives. Liberal views, but still very acceptable to moderate/establishment Democrats. Also an all minority ticket would likely energize minority turnout, which could be pivotal in states like GA.
6. Gretchen Whitmer (this relies on her winning re-election in 2022, which I think she does): Governor of MI, has received high praise for her handling of coronavirus. Also on the more moderate wing of the party, however has wide appeal across the entirety of the party, similar to Cuomo.
I'd say these are more-or-less the top 6 choices this far out, and here's a quick synopsis of why each choice is good.
So, other than Cuomo (blech), this list actually aged pretty well. Though I'd probably move Buttigieg up and Abrams up too, updated list:
Buttigieg (while he didn't get a foreign policy spot, given Biden's emphasis on infrastructure, Transportation is actually a pretty important job in the admin)
Beshear (see above)
Abrams (see above+flipping GA and leading the fight against the new voter suppression bills)
Baldwin (see above)
Whitmer (see above)
