Oof, good question. I think it'll ultimately be a boring, non-controversial white guy. So I could very quickly eat my own words because Shapiro fits that bill. The 2 that seem to be picking up steam are the governors of Kentucky (Andy Bashear) and North Carolina (Roy Cooper). Also Mark Kelly from Arizona. They're all fairly centrist and have won in Republican leaning or "purple" states which should theoretically help in the battleground states.
I can't see them pairing someone from the same state (California, so Newsome or someone of that ilk) or - and as stupid as this sounds and I absolutely hate that this may play into it - another woman (Whitmer), someone of color (Wes Moore from Baltimore who received some fanfare after the bridge fiasco) or Buttigieg, because he is gay and unfortunately doesn't poll well with some key demographics the dems will need to win. Which is a bummer because I'd prefer someone younger and exciting with good ideas, which I think Moore and Buttigieg generally are.