Postcards from Another World looks at the "Alternative Exits" - the stories where in our world the cast remained constant - and asks what might have been. In each case we will consider who would have been the most likely regular to leave, for on or off-screen reasons, and if such a thing is plausible at all.
An Unearthly Child
Any character that doesn't make it past The Firemaker isn't really ever a regular character, just a guest role for this one story. While it is next to impossible to imagine the Doctor leaving Ian or Barbara behind in 100,000 BC, it is even harder to imagine the series surviving for long if he had.
Could Barbara have plausibly stayed behind with the Thals? Her departure so early in the series might mean we would be looking back at the early years of Doctor Who and not think of the pairing "Ian and Barbara" quite so readily. Would it have given a larger role to Susan in her place? Or would it set a precedent for a much higher turnover of regular actors than we are used to, with the Doctor's Companions coming and going every dozen episodes (two-to-three stories) or so?
The Edge of Destruction
With nowhere to go except the TARDIS, who could possibly have left in this story? The only way out would be by death, perhaps at the hands of Susan's scissors. Even if caused accidentally, or while the characters were not in control of their own actions, could the series long survive the death of a regular this early?
Perhaps only now is the series mature enough, at 20 episodes, for the departure of a regular character not to jeopardise the stability and long-term survival of the show. Perhaps the most likely candidate for leaving here is Susan, who can stay with her new friend Ping Cho. Following in the footsteps of both Susan's grandfather and Marco Polo, they become explorers together.
The Keys of Marinus
The first example in the series of the Doctor's visit leaving a society in chaos and needing to be rebuilt, this would be the first example of the occasional trend for Companions to leave to assist in the rebuilding of the society (see Steven Taylor in The Savages as our world's first example of this). Ian remains on Marinus with his new friends
For any of the regulars to have remained behind in any safety would have required a significant change to the ending of the story (i.e. more than just a "I've decided to stay. Bye, Doctor.") and, as tempting as it may be to imagine, for Barbara to have remained would have required the rewriting of more than just one line of history. But, as surprising as it may seem, I would say the most plausible of the four regulars to remain behind here would be the Doctor, passing the TARDIS keys to Susan and staying with Cameca.
While it is possible to write Ian or Barbara out here - choosing a life in Central City on future-Earth over the uncertainty of further travels in the TARDIS - it is easier to imagine Susan as the first to depart (probably because we are so close to the point of her real departure in only three stories' time), with her being left in a unique position to help the humans and Sensorites come to a better understanding of one another.
The Reign of Terror
With them finding revolutionary France a source of constant danger, it is hard to imagine any of the regulars voluntarily staying here. While it would have been shocking to end the first season of the show with a death, if we are forced to choose one of the four to leave the series while still alive then perhaps the Doctor is once again the one to go for - this period of Earth's history was supposed to be his favourite, after all.
Planet of Giants
It's virtually impossible to imagine a regular leaving in this story except through death, since they are miniaturised the entire time.
"One day, I shall come back. Yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs and..."
"Grandfather, I'm only six inches tall!"
"Oh please stop bothering me."