Ryan Britt has a fascinating speculative piece in Inverse on the possible expanded role of Lady Margot Fenring, who only briefly appears in the original novels, but plays an important part in the story.
In Frank Herbert’s novel Dune (1965), the first mention of Lady Margot comes fairly early. On page 116 (Kindle Edition), Jessica finds a note left for her by Lady Margot, which, in code, warns Jessica that some kind of internal betrayal is coming. Jessica knows Lady Margot is a fellow Bene Gesserit and the wife of Count Hasimir Fenring (a sort of Imperial lackey who hangs out in the court of Shaddam IV, who lived on Arrakis with Margot prior to the events of the novel).
So, her first “appearance” isn’t really an appearance at all. Instead, like Princess Irulan or the Emperor, Herbert introduces this character “off-screen” before we actually meet her. But, it’s not like her role expands significantly after that. In fact, we don’t actually meet Lady Fenring until much later, on page 521, at the start of what is now labeled Chapter 35. (Herbert didn’t actually use chapter numbers in the text.)
Lady Margot is hanging out on Giedi Prime — the homeworld of the awful Harkonnens — watching Feyd participate in some rowdy gladiatorial contests. She’s also on a secret mission: The Bene Gesserit have sent her to obtain Feyd’s genetic material. Yes, that means she’s meant to seduce Feyd to become impregnated with his baby.
After that … Lady Margot’s story in the original Dune novels pretty much is over. The only appearance of Margot’s secret daughter, Marie Fenring, didn’t occur until the 2008 book Paul of Dune, written by Brian Herbert and Kevin J. Anderson. And it’s unclear if we ought to consider the Herbert-Anderson Dune books canon. Even the decidedly extra-canonical 1984 book The Dune Encyclopedia points out, “One question regarding Lady Margot remains: What became of her daughters, particularly the one sired by Feyd-Rautha Harkonnen?”
As Villeneuve has already stated in interviews, he’s making a point of expanding the roles of female characters in his film adaptions, and given the fact that an A-list French actor is playing Lady Margot (Lea Seydoux), it certainly sounds like we’ll be learning a lot more about this Bene Gesserit character in Dune, Part Two.