The plot of "Dune Messiah" involves the Bene Gesserit trying to manipulate the bloodline to keep themselves ingratiated in the highest circles of power. Paul has to balance the construction of a new empire while carefully avoiding his already-assigned messiah status. The story also sees the resurrection of Duncan Idaho (played by Jason Momoa in Villeneuve's film), since clones — or ghola — are common in this universe.
Villeneuve has been teasing a film adaptation of "Dune Messiah" for years, and it looks like he'll get his wish. He has already begun writing the screenplay. Whether he'll require two films to cover the events of Herbert's novel remains to be seen.