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.
Put it this way, if you were filming Sarah Jane Adventures, and you knew Elisabeth Sladen wasnt going to be avabile for a few episodes, you would probaly think to write an episode where Cylde & Rani, meet up with Jo & her grandson, somewhere outside of London.
Its certainly an option, but obviously the BBC, RTDs and everybody involved needs to think long and hard about if they should do it, because in some respects it would be a very easy thing to do, but maybe not the right thing.
To even have that option to consider, is very unlikely.