Interestingly, in one of the prequel novels, She and Allan, Haggard crossed-over his two most celebrated characters, Allan and Ayesha, the immortal white goddess-queen of the lost African city of Kor.
In The League of Extraordinary Gentlemen by Alan Moore and Kevin O'Neill, it is revealed that Quatermain's death in Allan Quatermain was falsified, and that Quatermain had returned to Europe in secret, having become reduced to an opium fiend once his supply of taduki ran out. Quatermain becomes part of the British government's secret team and helps to battle "the Devil Doctor" and H.G. Wells's Martian invaders. In a subsequent adventure, only briefly outlined, Allan and Mina Murray return to Kor and recieve the gift of eternal youth and immortality (which requires the elderly Allan to fake his death and take the identity of his non-existent son Allan Jr.)
According to Haggard, Allan's father emigrated to southern Africa after the death of his wife and three eldest sons. Allan was twice married, and had by his second wife Stella had a son, a medical student named Harry, whose death precipitates Allan's return to Africa from retirement in England in Allan Quatermain. Eminent pulp scholar Rick Lai has examined Quatermain's life story in an essay in his book Daring Adventurers, working out a chronology and suggesting that Harry may actually have been Allan's grandson. Wold Newton scholar Mike Winkle has also revealed that Allan had a daughter named Branwen by his first wife Marie, and that she in turn was the mother of the archeaologist Dr. Henry Jones, Sr.
- "The Mystery of Harry Quatermain and Other Conundrums" from Daring Adventurers by Rick Lai (online version of essay)
- "Watching the Detectives" by Brad Mengel (cites Mike Winkle's theories)