He's probably not going for a straight white guy unless something really extraordinary happens. I really have no idea how the Democrats' identity politics focus would deal with a Biden/ Buttigieg ticket.
Tammy Duckworth tops the list for most people’s potential running mates and she should obviously be included on Biden’s. From a key region but her Senate seat would still likely be safe if she gave it up, female, wounded veteran, minority. She checks all the boxes. No ideological heresies for the base to rage about
Yeah, she seems to look the best on paper.
Harris has the highest chance of being his pick. I'm certain that he will pick a female Senator, but if that person is not Harris, then I can also see: Baldwin (though he may be risking a Senate seat with her), Cortez Masto, Duckworth, Klobuchar, or Warren.
This would depend on how Harris does. She balances him in a few ways, and would be seen as a plausible President. If she's a strong second or third in the primary, it could also be part of a unity ticket.
If Booker, Klobuchar, Castro, or Gilibrand finish in the top three, they might easily become the favorites for running mate.