SPOILER ALERT: Up to and including "The Time of the Doctor" (2013 Christmas special)
Having seen all of the modern Doctor Who (9th Doctor onward) and bits and pieces of the classic series, I have a lot of respect for the original series (yes, the special effects suck, but for the 60s, 70s, and 80s it's pretty good). I love the little nods to the classic series as well (like the 11th Doctor having the Seal of Rassilon in "The Time of the Doctor" after having taken it from the Master in the Death Zone as the 3rd Doctor in "The Five Doctors"). I loved seeing Sarah Jane Smith in the 10th Doctor era, linking old Who and new Who together. In fact I'd love to see more of that, old companions returning. So here are my top 7 Classic Companions I would like to see again (Note: I've only included companions that it would be viable to see again, which would exclude most companions whose actor is deceased).
1. My number one choice is Romana. In the 4th Doctor era we saw two different versions of the Time Lady Romana. A third Romana and another known as Trey (some people believe this is the same incarnation) appeared in prose and audio adventures of the 8th Doctor. Especially with the expected return of Gallifrey in the next few years, we could easily see a new regeneration of Romana, perhaps a return of the 2nd Romana before her regeneration into the 3rd Romana, who the 8th Doctor meets.
2. I would love to see more of K-9. The mechanical dog appeared alongside the 4th Doctor in the classic series, later returning alongside Sarah Jane Smith in "School Reunion" and appearing regularly in the Sarah Jane Adventures. In my opinion, this could also be a good opportunity to have Luke, Sarah Jane's son from her spinoff, return and tie up any loose ends from the spinoff, since the show abruptly ended as a result of the death of Elizabeth Sladen.
3. Having recently watched a few episodes of the 7th Doctor with Ace, I would love to see Sophie Allred reprise her role as the sassy, explosive-loving woman that she was (only older now). In addition to simply enjoying her personality, I think this would be a great way to tie up some loose ends from the last episode of the classic series "Survival" and the 7th Doctor's return in the TV movie. Somewhere along the way Ace left the Doctor and I'd be interested to learn why. Russell T. Davies stated that Ace would have appeared in the Sarah Jane Adventures, but due to the unfortunate death of Elizabeth Sladen, that didn’t happen.
4. Seeing as the actor is still alive, I would love to see Ian Chesterton return. Ian traveled with the 1st Doctor and was, in a way, the original hero of the story, since the Doctor didn't get into a lot of physical action. It was referenced (if you were paying attention) in the 50th anniversary special "The Day of the Doctor" that he still works at Coal Hill. However, I might think it better to set an episode with him in the future, seeing as Sarah Jane references a rumor in "The Death of the Doctor" that as of 2010 Ian and Barbara hadn't aged since the 1960s.
6. Though arguably not part of the classic series, I would love to see more of Dr. Grace Holloway, from the 8th Doctor's TV movie.
The TV movie has a bad reputation, but I enjoyed it. Taking into account that Grace apparently had some other adventures with the 8th Doctor in the comics, I think featuring Grace would add some depth to the 8th Doctor that I wish we had.
7. Admittedly I haven't seen many classic episodes of her, but I would love to see more of Jo Grant (married name Jo Jones). She worked with the 3rd Doctor and later reappeared at his supposed funeral in "The Death of the Doctor" of the Sarah Jane Adventures. It was revealed that she has traveled the world and that she has a family of her own (including grandchildren). I think it would be cool to see Jo interact with the 12th Doctor, who reminds me a lot of the 3rd Doctor.
There you have it. Those are my seven returns that I wish Moffat would make happen. Which classic companions would you like to (realistically) make a return? Why?