I still think he would pick someone more moderate than he is. He also might have to pick a minority at this point. Cory Booker fits both criteria. He might be too moderate to agree to it though.
If Cory Booker says no, Anthony Foxx (former Mayor of Charlotte, Secretary of Transportation), and Ron Kirk (former Mayor of Dallas/ US Trade Representative) might be African-American alternatives. Loretta Lynch would add a lot of diversity, and emphasize the Obama administration's approach on civil rights.