A Newsom-Harris ticket would be an all CA ticket. Traditionally, regional affiliation as well as group affiliation has been taken into account when choosing running mates.
Newsom would probably prefer to keep his powder dry until 2028.
Illinois governor JB Pritzker has been mooted as a potential Democratic candidate over the years. He is the heir to the Pritzker fortune, and has seen off a MAGA challenger by a wide margin. Again, he might prefer to wait this one out, and possibly end up as a VP candidate.
Senator Tammy Duckworth of Illinois (double amputee, Iraq war veteran, with Thai-American ancestry) might be persuaded to give it a go. Her senate seat would be up for grabs, but Illinois usually returns two Democratic senators.
It would be absolutely extraordinary to dump Biden at this point - it would take a major health event (heart attack, stroke, death) to male dropping out look credible. The Dems would then be left with Kamala Harris, who could also be replaced (has the charisma of a plank).