I love Jo, but the version of Jo portrayed in "Death of the Doctor" seemed a little too flighty and dotty to fill Sarah Jane's shoes.
How about a show featuring a team of former companions from different eras of the series getting together, forming an organization to protect the planet when the Doctor isn't around? If anything, I've always found it hard to believe they haven't made more of an effort to seek each other out, at least socially. I mean, who else could they talk to that would understand what they've been through?