Doctor Who has been around for a long time, with 39 seasons between both iterations. The show has gone through a lot of changes over the years, but some patterns remain the same. One of those patterns is the doctor taking on a companion in their travels. Sometimes it is a group of companions, but they always have at least one person traveling or regularly working with them. Sometimes they are human, other times they are aliens or robots. Many come from Earth in the present day, but sometimes they are from the past or future.
Companions come and go, and sometimes come back. Sarah Jane Smith returned for "School Reunion" in Series 2 of New Who, and evem got her own spin-off show which saw reappearances from Jo Grant and Brigidier Lethbridge-Stewart. Captain Jack Harkness got to star in Torchwood and made several guest appearances in later seasons. But there are still some Doctor Who companions whose stories have not been touched, that could have great potential for a return.
5 Susan Foreman
When Doctor Who first began, a lot of the existing canon had not yet been firmly established and the Doctor (presumed human at the time) was traveling with his granddaughter, Susan. Her only big appearance after the William Hartnell era was reprising the role for the special The Five Doctors. The doctor has had many companions over the years, but seeing an actual relative is very unusual. As the Doctor's biological grandchild, Susan is a Time Lord herself. And she stayed on Earth after leaving the Doctor, so she may not have even been present for the Time War. And even if she was, well, other timelords managed to bounce back.
Susan's original actress, Carole Ann Ford, is still alive. Of course, she is over 80 now and may find it too demanding, or the production team might prefer a younger actress. But the advantage of Timelord characters is they can regenerate. A new actress doesn't even have to look like Ford. It does not even have to be an actress. What's more, regeneration can change a Timelord's sex, meaning Susan could even be played by a man. Imagine a male Susan meeting a female Doctor — that might be an interesting dynamic.
4 Romana
When the Doctor was given the mission to find the Key to Time, he was also given an assistant named Romanadvoratrelundar, whose name he insisted on shortening to "Romana." Like the Doctor, Romana is a Time Lady and proved quite capable. She was smart, witty, and good at solving problems. But she more or less disappeared after leaving. Romana had a cameo in The Five Doctors (through archive footage) and actress Lalla Ward reprised the role for the animated version of Shada, but she has not made a real appearance since the Tom Baker era.
Romana has the same advantage as other time lords on this list: she can regenerate. In fact, she already went through one regeneration, being played first by Mary Tamm and then by Lalla Ward. It may be possible to get Ward to reprise the role, but failing that, she can be played by an entirely different actress or actor. There are also some unanswered questions regarding her whereabouts with regards to events of the current series. They never state whether she was affected by the Time War, or any of the subsequent catastrophes caused by her kind. It's entirely possible for her to make a comeback.
3 Leela
Despite only being a companion for one season, Leela is well-remembered among fans and stands out among them for her unusual circumstances and characterization. She was originally a warrior from a tribe called Sevateem, which turned out to be the descendants of a spaceship crew that crash-landed on an alien planet. The Doctor's presence would call into question everything she was brought up to believe, and following him gave her a chance to see that the universe was much more complicated than she could have imagined. Being an expert warrior and huntress proved useful on multiple occasions.
In Leela's last appearance in The Invasion of Time, she chooses to stay behind on Galifrey. That opens up a few potential storylines for her return. What has she been doing on Galifrey since then? Was she involved with the Time War? Spending so much time on the Timelord homeworld may have even given her time to learn how to regenerate, making it possible to recast if Louise Jameson declines returning. Plus, it would be interesting to see how she reacts to a non-Tom Baker doctor. There are some really good opportunities here.
2 Adam Mitchell
Possibly one of the more forgettable companions of the series, Adam only travelled with the doctor for two episodes in the 2005 season with Christopher Eccleston. He first appeared in "Dalek" before accompanying the doctor in "The Long Game", which ended with him being kicked out of the TARDIS. As a companion, he would not have much to offer. However, he might have some potential to return as an antagonist.
The reason the Doctor kicked him out was because he tried to abuse the opportunity to visit the future. Adam's plan was to gather data on important discoveries in the future and send them back to his present so that he could then plagiarize those same discoveries to make a fortune. When the Doctor found out, he quickly thwarted those efforts, and decided Adam couldn't be trusted with Time Travel.
Adam's planned con was a pretty clever one, and he has a bit of technology already: that strange opening in his forehead. He might not have access to time travel, but that might not stop him from coming up with similar schemes or finding a way to use the knowledge he already has to his advantage. It offers a pretty big recipe for disaster, one that could cause any number of problems for the Doctor. Plus, he might have some deep-rooted resentment building up from being abandoned by the Doctor and a desire for payback that might make him do something desperate like help the Master.
1 Sgt. Benton and Captain Yates
Beginning in the Patrick Troughton era, the Doctor became a regularly collaborator with UNIT, a top-secret division focused on addressing extraterrestrial threats. Most famously, he worked with Brigidier Lethbridge-Stewart, who became a regular character through the John Pertwee series. Nicholas Courtney reprised the role a few times, making guest appearances in the Tom Baker and Sylvester McCoy eras. The Brigidier managed to return for The Sarah Jane Adventures, but unfortunately Courtney died before he could return to Doctor Who.
But the Brigidier was not the Doctor's only friend on UNIT, which has made its way into the revived series. Aside from the Brigidier, the doctor had two other friends on UNIT: Sergeant John Benton and Captain Michael Yates, both of whom frequently played a role in stopping major catastrophes. Even better, both actors are still alive. They might not be able to bring back the Brigidier, but they can bring back two of his closest friends.
This could add some interesting dynamics to a future story involving UNIT, considering both men have a lot of experience dealing with some of the Doctor's worst enemies, including Daleks, Cybermen, and the Master. It would also be cool to see how they interact with the Brigadier's daughter Kate Stewart, possibly acting as mentors for her.