With Joe immediately endorsing Kamala, it would be hard for the elites to bully another onto the ticket over her. Not impossible though.
Harris / Buttigieg is probably my top choice right now... Harris / Shapiro is another option.... Harris / Romney would be fascinating, though I think it's probably too far fetched (and I don't think Mitten would want the job).
If she's not on the ticket, I would think it's Whitmer, with one of Wes Moore, Stacey Abrams, or Raphael Warnock.