Warning: Spoilers ahead for Doctor Who season 15, episode 7, "Wish World."David Tennant's Tenth Doctor, Doctor Who has been resistant to introducing of the Doctor's family - that is, perhaps, until now.

Doctor Who season 15, episode 7, "Wish World," ends with Ncuti Gatwa's Fifteenth Doctor theorizing that Poppy is actually his real daughter, rather than their relationship being part of the worldwide illusion. While this would be a fascinating twist, it's long been obvious that the Doctor has had, or will have, children - and that they will also have children. At the very least, the Doctor's offspring reproduce at least once, as proven by Susan's existence. What the show has never revealed is what Susan has done since she parted ways with the Doctor, but "Wish World" provides a brilliant tease.

Doctor Who's Conrad Hints That Susan Had A Child After Leaving The TARDIS

Conrad's summary of why companions leave the TARDIS seems too coincidental

William Hartnell's First Doctor locked the TARDIS doors on 23rd-century Earth and left his granddaughter behind. Susan had fallen in love with a man called David (Peter Fraser), and the Doctor was abandoning his only known relative out of kindness, knowing that she would also choose her grandfather over her own happiness. Although she returned in 1983's "The Five Doctors," Susan's life after her original exit has been referenced incredibly rarely, but Jonah Hauer-King's Conrad Clark may have provided a clue about her new family in "Wish World."

Acting as Earth's resident storyteller, Conrad reads everyone the story of a character he refers to as "Doctor Who," who is quite obviously the Doctor. While mentioning the string of companions who have traveled in the TARDIS with the Doctor and have since left him, Conrad explains that the companions' reasons for leaving the Doctor often include love, marriage, and children. Susan did leave the Doctor for love, and given how recently she returned to the show, it feels a little too coincidental that Conrad would be referring to anyone more specifically than Carole Ann Ford's character.

"The people who lived on the Earth were called humans. Doctor Who had lots and lots of human friends. They loved him and traveled with him. But sometimes, they'd leave him, so they could go and fall in love and marry and have babies. And then the Doctor was all alone."

- Conrad Clark in Doctor Who season 15, episode 7, "Wish World."

If we assume that Conrad is referring to Susan, then the implication is that she and David got married and had a child together after she left the TARDIS. The said child would be the Doctor's great-grandchild, making the Doctor a great-grandfather. If Poppy does turn out to be the Doctor's daughter, then this would make her a grandmother. Considering how rarely the topic of the Doctor's family has arisen through the decades, this would be a huge influx of information on this topic.

Doctor Who Already Set A Precedent For The Doctor Being A Great-Grandfather

Other Doctor Who projects have introduced Susan's offspring (but they're not canon yet)

The Doctor Who franchise expands far beyond the show, but only other live-action projects and spinoffs like Torchwood and the War Between the Land and Sea are true contributors to the universe's main canon. Other mediums, like audio adventures, comic books, and novels, contain some great Doctor Who stories, but unless they're acknowledged by Doctor Who and its spinoff shows, then they exist in a different continuity. However, a Doctor Who audio adventure has already added to the Doctor's family tree via Susan.

Carole Ann Ford has already reprised her role as Susan in several projects by Big Finish Productions, voicing the character as the extended universe fleshes out her life after the Doctor. In these alternate adventures, Susan has already had a child with David, so the idea isn't brand new in the larger franchise. Therefore, Whovians who engage with the saga beyond the TV shows may not be too surprised if it turns out the Doctor is now a great-grandfather, as the twist has already been toyed with in other Doctor Who stories.