I kinda think that K-9's return might be part of a set up for a pseudo-reboot of the very popular "Sarah Jane Adventures", with Clara taking Sarah Jane's place.
Keep in mind that it was very popular show, and CBBC hasn't found anything nearly so popular to replace it. I'm sure they'd love to revive the show, particularly now that an appropriately respectful span of time has passed. The concept is still sound, they apparently had plenty of unused scripts already in the pipeline, the only thing missing up to this point, was finding a worthy replacement for Sladen. I think Coleman might be the one. The fact that she supposedly is extremely popular with younger DW viewers doesn't hurt either.
Of course, there'd have to be a whole new crop of kid assistants, with one of the now grown-up "kids" from the original series lending a hand, for continuity sake.
Jenna Coleman and Daniel Anthony are nearly the same age. (Perhaps Clyde will stick around to help out, and we'll eventually learn that his mum's maiden name is "Pink"?)