He can't run again. He said in 2020 that he was only running for one term to get rid of Trump. To go back on that, especially considering his obvious mental acuity issues, would be a big mistake for the Dems since many independents and undecideds could very well vote for a DeSantis simply on the basis of Biden not being up for it.
Newsom would probably be the best choice at this point. He's very sharp and could easily riposte against Republican attacks on social media and in debates. After the fall of Cuomo, I can't see another Dem at the moment who could muster such a good challenge. Kamala has done nothing as VP to improve her position relative to 2020 (although she has bargained with Newsom before and he might not run if she does sadly). Buttigieg, the corporate darling, might throw his hat in as well but I don't like him nearly as much as Newsom and he still doesn't have a whole lot of experience.