You are right about the running mate problem. If Joe picks a formidable candidate, it’s going to be a tough fight.
I haven't seen or heard one, and there's not a hint anywhere they have one hiding in the wings to spring on us.
If he had not painted himself into a corner by announcing it would not only be a woman but an African American woman (I guess it can't be Kamala b/c she is not African haha), he might have been able to pick a "formidable" VP. Given the various lists we have all seen, I don't think any of them are any good.