My tentative list right now (just focusing on the publicly available candidates being vetted that haven't refused):
Kelly - he was created in a lab for being a vice presidential balancing candidate. Downside is the key Arizona Senate seat would be up for special election in 2026. That's a big downside. He can help Kamala Harris on immigration, which will probably be her weakest issue.
Cooper- he probably is the favorite at this point as he has the best pre-existing relationship with Harris. It is hard to do well in North Carolina these days and he's been a star. Downside is he would be a great candidate to win the North Carolina Senate seat in 2026. And it is one of the few seats in 2026 that Democrats could pick up.
Shapiro- great speaker in an essential swing state. He's super well-liked in Pennsylvania too. Downside is his statements on Israel. I'm not sure how that would all shake out, but his swing state potential here is too big to ignore.
Beshear- good communicator who is well-liked in a red state. He is the perfect opponent to JD Vance. Downside is that a lot of his popularity is Kentucky specific though and Kentucky isn't a swing state.
I think the bench is really deep this cycle, so I'm not stressing this too much.