Standard political practice would be a governor—of a decent size swing state if possible.
These days only two Democratic governors meet that criteria—PA and MI. MI has the “legend” of the phony baloney FBI planted attempted kidnapping. Democrats eat up those lies—and love the “brave woman” mythos.
CO would be just outside that window imho—but a possible compromise choice in a divided convention.
CA probably wants it—and might try to elbow his way into the job. He has the delegates to cause quite a bit of chaos on the convention floor.
Conclusion: Whitmer is the most likely choice. The Democrats wish she were black but they may have to “settle” this time.