Pres. Biden squandered 3.5 years by choosing not to mentor and bring along possible successors, in addition to VP Harris. I don't doubt VP Harris will get the top job nod from Democrats at the convention.
Mayor Pete? I like him, what I heard and saw of him when he ran in the prior Democratic Party primaries won by Joe Biden. However, is he capable / qualified to assume the Presidency if something happened to a President Harris on Day 1? No, I don't think so. I don't think the nation is prepared to knowingly elect a homosexual at that level of office, either.
Pundits are already mentioning former Naval Aviator / Astronaut and current Arizona U.S. Sen. Mark Kelly and PA Governor Josh Shapiro as front-runners for the VP nomination. Either of these two would be solid choices and add to the entire ticket.
I don't think a two-woman ticket would be successful, though there are a couple of well-qualified women who come to mind: Michigan Gov. Gretchen Whitmer, Minnesota U.S. Senator Amy Klobuchar. I'd have included former disabled helicopter pilot / Illinois U.S. Senator Tammy Duckworth but I think there are questions about her birth in Thailand to an American father and whether or not she meets the qualifications because of that (i.e., foreign born).
Democrats have a very deep bench of possibilities, had only President Biden not squandered all that time after saying he'd be a transitional President.