For the record, a mere three presidential nominees in the last forty years have picked another presidential candidate as their VP (Reagan, Kerry, Obama). Not to say Biden won't, because there's picks that line up with what he needs, like Kamala for instance, but it's normally been a non-candidate choice from the Senate, Governors or more rarely the House, which fills in some gap in their appeal or shores up a demographic they need to win.
In this case, Biden will need someone much younger than him, and experienced in public office because his age means it's more possible than usual that they may have to become president during his term, and whoever it is will be the natural choice to run in 2024 or 2028. Look for at least 3 of these 4:
- Much younger
- Female
- POC
- More left