Prophecy centers entirely around the origins of the Bene Gesserit, known only as "the Sisterhood" in the show. However, there's a very simple reason for that. "The Sisterhood at its inception is not called the Bene Gesserit,

The biggest problem is the problem that plagues seemingly all prequels: the fact that no matter how far back in the past the story might go, imaginations can only stretch so far. Once again there is a conflict between the Harkonnen and the Atreides families (the novelty here being the Harkonnens get more screentime),

Dune: Prophecy's Jade Anouka teases Sister Theodosia (Theo)'s devotion to Valya, her mysterious past, and how she'd run the Sisterhood.