Raphael Warnock or Ruben Gallego if he's elected Senator. Jon Ossoff is a good suggestion. It should be someone from the Sunbelt. Ideally a POC.
Gallego's an interesting name drop, and I have to say, I agree. Instead of focusing on just AA's when we talk about Dems driving up turnout, I think a Hispanic running mate could do wonders in reaching out to the Hispanic community, reversing the R trend, etc. Someone with Gallego's background may help lock up the Hispanic vote and increase turnout with Hispanic Democrats - after all, if the Democrats win, it'll be another historical first, this time for Hispanic-Americans.
Gallego would help out in AZ and NV, both of which are pivotal swing states (and the trends in both are not nearly as one-sided as those in GA, which is naturally trending further and further left). He might also help put TX in play, although realistically I think it's fool's gold for the Democrats this year.