Companions have left the Doctor on their own accord or met a much more tragic end.
Some stories we saw on screen, some we didn't.
Doctor Who: 19 Times Bad Wolf Connected the Doctor and Rose Tyler
Here are some Doctor Who companions and characters we believe should have another go in the TARDIS, or perhaps healm their very own spin off! Spoilers, they aren't all investigative journalists with robot dogs.
Luke Smith: K9 and Co
Luke Smith: Sarah Jane Smith's adopted son in The Sarah Jane Adventures. Grown from DNA by the Bane, he's not fully human and would be a big target for aliens on his own. He shared adventures with Sarah Jane and his mates, Maria Jackson, Clyde Langer and Rani Chandra in the spin-off show, which was sadly canceled after the untimely death of Elizabeth Sladen. He has the brain of a million humans and K9, his trusty robot dog to guide him through college. He'd be carrying on his mum's legacy and perhaps we'd see the Bannerman Road gang make a reappearance.
Rose Tyler and Ten 2: Parallel Love
Rose Tyler and the Metacrisis Doctor, created by Donna Noble, were forced to a parallel universe on the Doctor Who Season 4 episode "Journey's End." This version of the Doctor committed genocide against the DALEKS, and the real Doctor wants Rose to help him learn compassion. He's human and will age at the same time as her. Roses' family also lives in the alternate universe, and her mother just had a son. From growing a TARDIS to fighting cybermen, the possibilities are endless! They may even join parallel Torchwood.
Jo Grant: Travels of the World
Jo Grant traveled with the 3rd Doctor in classic Doctor Who and made a return on The Sarah Jane Adventures episode "The Death of the Doctor." She was originally an assistant, given the job of looking after the Doctor by the Brigadier. Her memories of traveling the world helped save the day on The Sarah Jane Adventures, overloading the system the Sanseeth were using to create a TARDIS key through Sarah Jane and Jo's memories. Through this, they could control the blue box. During her travels, she also promotes humanitarian and ecological rights. Jo ended up married to Cliff Jones and had seven children. She even traveled down the Amazon River in a tea chest!
The DoctorDonna: What Could Have Been
Let's be real, Donna Noble deserved a better ending. After helping save the universe on the Doctor Who Season 4 episode "Journey's End," towing the Earth back home and becoming part Time Lady, the Doctordonna was forced to have her mind wiped because the power was killing her. The DoctorDonna may have had her memories erased, but if she had lived, she would have made an amazing TARDIS team with the 10th Doctor. She'd be able to keep up with him with her superhuman brain and the Doctor would have his best mate at his side. With powerful aliens and comedy every week, it would be a guilty pleasure like tv shows human Donna enjoyed.
Missy (The Master): MISSY Who
The self-proclaimed queen of evil has a difficult relationship with the Doctor, childhood best friends turned best enemies. The spin-off would tell us how the Master became Missy and escaped the Time War, which was never shown on screen. The last we saw of the Master was after his future self, Missy, killed him and he was sent down in the lift on the Doctor Who episode "The Doctor Falls." It would explain how he escaped the burning ship and ended up on Gallifrey. It would show how she turned from ruthless killer to having compassion for the Doctor. Missy also has unfinished business with Rassillon, leader of the Time Lords, and may run into her past self again.
Mickey Smith: Pete's World
Mickey Smith was originally Rose Tyler's boyfriend in Series 1, but joined Team TARDIS in Doctor Who Series 2 after a suggestion from Sarah Jane Smith. He stayed in Pete's Parallel world for a while in Doctor Who Series 2 and joined a cyberman fighting gang after his counterpart Rickey was killed. We're sure he helped Rose's father learn to care for his parallel daughter, and took care of his grandmother when he wasn't going in guns blazing on the old Torchwood.