It would be tough for any Democrat to beat an incumbent Republican Governor in Indiana. It's a state Romney won by ten points, and Trump won by almost 19. Holcomb also remains broadly popular.
https://morningconsult.com/2019/01/1...rnors-q4-2018/
I could see Biden running with Abrams given her high profile, but Gillum and the mayor of Flint, Michigan probably don't have the resume for Veep. Buttigieg might elevate himself sufficiently in the primaries, although I don't know if a female candidate would pick a gay guy as a running mate.
The overwhelming majority of recent Democratic running mates have been Senators, which makes sense for a party that prioritizes the federal government and ensures that the #2 should be familiar enough with national issues to avoid embarrassing themselves. They might go in a different direction, although Beto and Julian Castro have similar advantages. I'm guessing the Democrats want balance on the ticket, so it's unlikely to be two white guys, two women or two minorities (there is a further argument that it's unlikely to be two white people.) Klobuchar would've been a good fit for some of the male candidates, but the staff abuse stories have hurt her.
Potential tickets...
Biden/ Tammy Duckworth (She's a midwestern Senator who served in the military, and adds diversity to the ticket.)
Biden/ Kamala Harris (This could be a unity ticket following a bruising primary.)
Bernie Sanders/ Amy Klobuchar (It's very tough to get a running mate for Sanders. He needs someone Democrats like who his base won't see as a sellout. Since he's Jewish and his only child was born out of wedlock, it would make sense for his running mate to be a Christian with children. Klobuchar would be a campaign surrogate in the midwest.)
Bernie Sanders/ Julian Castro
Kamala Harris/ Beto O'Rourke (Beto would be similar to Edwards in 2004, presumably without the personal flaws.)
Kamala Harris/ Sherrod Brown (Brown seems the closest thing to Biden circa 2008)
Kampala Harris/ Doug Jones (adds geographic diversity/ appeal to moderates) while doubling down on criminal justice, and appearing qualified enough without overshadowing her)
Beto O'Rourke/ Tammy Duckworth
Beto O'Rourke/ Kamala Harris
Elizabeth Warren/ Beto O'Rourke
Elizabeth Warren/ Julian Castro
Cory Booker/ Amy Klobuchar (He has explicitly promised to select a female running mate which limits his choices as he probably wouldn't pick a woman of color, or someone from the Northeast.)
Cory Booker/ Gretchen Whitmer (She's a swing state Governor from a politically important region.)