Mattis should be the first call for VP. Kelly should be second.
In no particular order, my short list
Whitmer (strong gov, intelligent charisma)
Shapiro (")
Newsom (points for visibility, but maybe an all-Cali ticket a bit much for middle America)
Buttigieg (for maximum Firsts per ticket)
Mattis (an honorable man in any universe)
Cheney (Unity tickets don't often win, but LIzzie could swing some Lincoln Project folk and other defectors)
Booker (this guy is fucking serious about affordable housing and has terrific energy)
Beshear (so I can keep making a terrible pun, as witnessed here earlier)