The best bet for the show to continue would seem to be Jo Grant.
She could bring her grandson along, too. And its Jones now.
Just finished watchin' SJA 'Death of the Doctor' for the first time, and I agree, if the series continues, Jo would be the best way to keep it goin', since the teenages know her already and they'd have a peer for her to bring along on adventures.
I understand how you feel, but seriously: Let's just let it go, eh?