Although 'The Day of the Doctor' (2013) showed representations of all of the actors to portray The Doctor up to that time, most were done using archival footage. With seven actors who portrayed The Doctor, "The Power of the Doctor" has the most appearances filmed specifically for one episode (Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann, David Tennant, Jodie Whittaker, and Jo Martin), plus David Bradley portraying the William Hartnell Doctor.
Ian Chesterton (William Russell) is the first companion from the first episode, “An Unearthly Child” (1963) and the entire 1960s era, to appear in the revival series. With a gap of 57 years, his cameo holds the world record for the longest gap between televised appearances as the same character.
From the TARDIS Wiki:
In 2022, Ian attended a meeting of many of the Doctor's past companions, including Graham O'Brien, Dan Lewis, Yasmin Khan, Tegan Jovanka, Ace, Melanie Bush and Jo Jones. He expressed surprise upon hearing Dan refer to the Doctor as "her".
I don’t take into account any of the stories from novels, comic strips, short stories, or audio dramas. There are just too many opportunities for contradictions and discrepancies. (See the original stories for ‘Human Nature’, ‘Blink’ and the comic book adventure dealing with the Meep.) But of all the adventures which I saw described in the TARDIS Wiki, this intrigued me….
From Wikipedia:
One BBC Books novel, “The Face of the Enemy” by David A. McIntee (1998), picks up the story of Ian and Barbara, now married to each other, in the early 1970s, the two of them collaborating with the Doctor's colleagues at UNIT and his enemy the Master when Ian is hired as the Third Doctor's temporary replacement while the Doctor is taking a trip away from Earth. In this book, they have a young son named John. Many of the novels mention a 1980s pop star named Johnny Chester or Johnny Chess, intended to be the same character. Chess is idolized by the Seventh Doctor's companion Ace and has apparently been romantically involved with the Fifth Doctor's companion Tegan.
I know it's photo-shopped,
but I have a soft spot for this picture.
I can’t state for certain that holds true for the televersions of Ian and Barbara, save for the being married part, of course. I think that has to be a given in all the fictional metaverses. Like a fixed point.
But as for the existence of Johnny Chess as a son of Ian and Barbara in a hypothetical untold story from Toobworld, I’ll just leave it unsaid. The possibility is out there.
Between the original series, the cameo at the end of the Thirteenth Doctor’s run*, and those videotapes recorded by UNIT regarding the adventure during the Crusades, Ian is covered for the minimum amount of appearances to qualify for Hall membership.
Barbara, on the other hand, has the original series and the references in the home video of "The Crusade" and that episode of ‘The Sarah Jane Adventures’. She needs one more entry to qualify and I’ve had that ready for years. (Remember – with the Birthday Honors inductees, “What I say, goes.”)
In several posts over the years, I’ve played with my splainins, tweaking them every so often as new ideas came to me or new information was available from the show.
When it came to the idea of bringing the characters back (especially after it was learned that they stopped aging in the 1960s), it was O’Bvious that while it was a sweet premise, it couldn’t really play out on the screen. We already saw that Russell was getting older in those narrative videos for “The Crusade”, but more importantly – and sadly – Jacqueline Hill had passed away in 1993.
I don’t know if it was ever suggested that the roles could be recast, but if so, I’m glad somebody put the (technobabble alert!) kibosh on that. In my Inner Toob blog, I have often let my displeasure for recasting be known. There are circumstances in which I don’t mind a different actor coming in to take over, but I don’t think any of them could apply to Ian & Barbara. (It’s bad enough that they recast William Hartnell with David Bradley as the First Incarnation, but at least a good splainin was provided for that when the Tenth Incarnation met the Fifth in the short, “Time Crash”.
TENTH INCARNATION to FIFTH INCARNATION
"The frowny face! I remember that one!
Mind you, a bit saggier than I ought to be.
Hair's a bit greyer.
That's 'cause of me, though.
Two of us together has shorted out the time differential.
Should all snap back into place when we get you home."
But humans can't regenerate. Well, except for River Song. It's one of the few splainins for a Terran recastaway which I can't use. And I wouldn't want to have the roles recast and leave it at that. It would have to be Jacqueline Hill and William Russell or forget it.
O’Bservation:
I accept that William Russell and Jacqueline Hill have “televersions” in Toobworld; they were played by actors in the TV movie which provided a behind-the-scenes look at the creation of ‘Doctor Who’ – “An Adventure In Space And Time”. From what I understand, both of those actors have gone on to play Ian and Barbara in audio dramas and I have no issue with that. Those take place in a different fictional universe.
So how do we splain away Ian aging if he was supposed to be immortal?
What if in some unseen adventure, fanfic if it must be, Ian journeyed back in Time to ancient Greece in the Age of Legend? Doesn't even have to be with the Doctor, no matter what incarnation you favor. There are plenty of Time Agents and other resources for reaching back to that time... the Time Tunnel! Why not? I am partial to crossovers, after all.
But if you want to keep it within the Whovian stratosphere, then I think traveling with the Curator might be the best bet. At some earlier stage in their lives, still an unseen adventure, Barbara and Ian gained their immortality. (It may be unseen, but perhaps not unread - I'm sure somebody has addressed the immortality question by now, either in some licensed forum or fanfic.)

However.... Something must have gone wrong in Ian's case. And it didn't become recognizable until he and Barbara, traveling with the Curator, arrived in ancient Greece, during the Age of Legend. Perhaps it was the work of a sorcerer or a warlock - using a spell aimed directly at Ian. He was still immortal, with ever-lasting life. But he wasn't granted ever-lasting youth. He became old, and frail, and withered. And among those who witnessed this phenomenon would be a story-teller who passed on the tale as myth, being the only way the people of his time would come to accept what must have seemed so inconceivable as Truth.
(Not too far off from what happened in the present day when the people of Earth Prime-Time easily accepted the emergence of the Cybermen from St. Paul's, the Army of Ghosts all across the world, the Battle of Canary Wharf, the Clan Slitheen attack on Big Ben, and the total removal of the planet from the Solar System by Davros and his Daleks, as being mass hypnosis, publicity stunts, whatever to keep them from acknowledging the Truth.)
But after they departed, that story-teller began weaving this new myth, one in which the sight of Ian shriveling down due to the rapid aging process and looking rather like an insectoid. And they would forever after be remembered by new names - as the goddess Eos and her lover Tithonus.
This is only my suggestion as to what happened. You might have your own. That’s the glory of fanfic.
Barbara was not affected by that aging affliction at all. She would be still looking the same as she did when she left the TARDIS with Ian all those decades ago. But they couldn't stay together forever with Ian in his aging condition. Eventually the Curator would have to do the only thing possible: he would have to bring Ian to the planet of New Earth where he could be tended by the Sisters of Plenitude. Since we saw Ian in 2022 at that support group meeting of former companions, it would have to happen at any point after that. (Death of an actor in the real world does not also signal the death of their character in Toobworld. The mention of Jacqueline Hill's Barbara Wright as still being alive seventeen years after Ms. Hill had died proves that officially.)
These are the musings I wrote about Barbara’s continued existence in Toobworld. In these blatherings you will find that one last credit which she needed for membership in the Hall….
Barbara should be considered as still living on Earth Prime-Time; she will never die and she will always look as she did in the early 1960s. I'd like to think that the Curator provided for her in some way, perhaps by setting up living quarters for her in a disused wing of the Under-Gallery. And she would not be wanting for company, as I'm sure that there would always be somebody from the Doctor's band of former Companions who could spend time with her. Who else could understand what a difference the Doctor made in their lives?
And more than likely, Barbara would be kept busy by UNIT. She could spend her time in research, or drawing on her skills as a teacher to impart her experiences to UNIT officials. And she might have taken on a lover or three - she would always love Ian, but she would have accepted that he was but a memory now; Life goes on.
2]
In an earlier post, I pointed out that Barbara had become a writer by the early 1970s, fictionalizing her adventures with the Doctor. Locked away in the Under-Gallery, I'd like to think she's there still, typing up new stories which she could sell. In fact, this idea just struck me - in my vision of Toobworld, it is Barbara Wright-Chesterton who is now the head of UNReel, the organization which is in charge in keeping the public from panic should they ever find out that the exploits of the Doctor, James Bond, the men from U.N.C.L.E., and others are actually happening. Better for the general safety of the world that they think it's all fiction.....
3]
I’ve made the claim in the past that ‘Columbo’ had a definite crossover with ‘Doctor Who.” It can be found in the episode “Publish Or Perish” when he is examining the poster for a book released by Greenleaf Publishing – MODERN AZTEC COURTSHIP PRACTICES.
The woman seen in the illustration of that cover is the artist’s rendering of Barbara Wright, one of the first Earthlings to travel with the Doctor. Along with fellow teacher Ian Chesterton, she accompanied the First Incarnation of the Doctor and his granddaughter back in Time to the era of the Aztecs. (And just to double down on the 'Columbo' connection, the artist was Max Barsini from the 'Columbo' episode "Murder: A Self Portrait".)
And it was during that time among the Aztecs when Barbara made note of the courtship practice which almost landed another wife for the Doctor because of the miscommunication regarding the offering of cocoa….
And there you have it, the reasons why Ian and Barbara (Wright) Chesterton are eligible for membership in the Television Crossover Hall of Fame.
Good night and may God bless you both.