10 Doctor Who Characters That Should Return
6. K9
K9 is such a beloved character. I mean, he's a robot dog for crying out loud. What's not to love? Plus he became such a staple of the Fourth Doctor's run, next to jelly babies, and a long multi-coloured scarf. It's just saddening that the robot dog hasn't made too many comebacks since his final recurring appearance in Warrior's Gate.
He had a failed spin-off attempt, before having minimal screen time in his reappearances in both The Five Doctors, and School Reunion. Even when The Sarah Jane Adventures came along he wasn't made a recurring character until Series 3, but was disappointingly sent away with Luke Smith when he went off to college at the start of Series 4. Also, the less said about his second spin-off the better.
Considering K9 has already had four models, it's an easy assumption that K9 Mark V could easily be introduced down the line. Either The Doctor encounters this latest model, or he just randomly decides to build it himself. It'd just be really nice having K9 aboard the TARDIS as a companion again, and hearing John Leeson's voice once more. Perhaps you could see the return of K9 Mark IV, who'd perform a similar service as Luke coming back, thus paying tribute to Sarah Jane Smith through the inclusion of her loyal robot dog.
Or, going back to my original suggestion of introducing K9 Mark V, wouldn't it be poetic if Sarah Jane sent The Doctor this latest model, similar to how the Time Lord originally did the same for her in K9 and Company. It could be a final gift from her in order to remember her by, as well as saying thank you for all The Doctor had done for her. That would be beautiful.