Both on the ticket can’t, in practice, be from the same state, due to an electoral college rule, especially if the state has a lot of electoral votes.
If Biden had died, or was ill, a lot of voters would want a ticket that honors his legacy.
Harris would probably get the nomination and should pick a normally qualified (current/former senator or governor) non-Californian who she personally likes. Being a moderate, I like Gina Raimondo.