The first few that jump to mind are Cory Booker or Julian Castro if she's looking for a man of color. If she's looking for a white man, Cal Cunningham, Andy Beshear, and possibly someone like Conor Lamb or Josh Shapiro if they win PA Gov/Sen in 2022. If she wants a white woman, Gretchen Whitmer and Tammy Baldwin seem like great options. And for a woman of color, which strikes me as the least likely scenario, then KLB, Stacey Abrams, Tammy Duckworth, and Michelle Lujan Grisham would be up there for sure.
In terms of Lamb, Shapiro and Abrams, it seems unlikely that Harris would pick someone elected in the previous cycle, two years not really time in the job to get credit for it or appear experienced.