Pramila Jayapal would be great but she's not a natural citizen.
Tulsi Gabbard.... i think is probably strangely high on Bernie's list, i wouldn't like it and i don't think she would bring him any new supporters, but i could see it happening.
Elizabeth Warren i think is exactly who Bernie wants and has wanted since he ran the first time, but i don't know if there is too much of a rift between them now.
Ayanna Pressley would be my pick. she endorsed Warren but idk how much that matters, presidents pick an opponent as their running mate often