Spoiler:
i think it's possible that rabban dies early. if not, he barely escapesi don't think that this film can't simply end with an hour+ of non-climax screentime. you can certainly get away with that in a drama (irishman, amirite?), and in the concluding act you can drag the post-climax out for infinity if you want (return of the king, amirite?), but you can't get away with that in act one of an action scifi. there is a substantial series of events from the time of duke leto's assassination, up until usul/muad'dib is named, and then until jessica takes the water of life. tbh i think leto will be dead @90min into the film, with another hour/hour and a half to spare.
I think rabban will be used as the sacrificial lamb to rectify this and keep with movie-making theory and tradition of an end-of-film climax and significant win, with muad'dib leading a fremen raid on some approaching invasion force or outpost which rabban is overseeing. sure they could just end it with their first raid on some harkonen spice harvesters...but where is the sense of fulfillment in that? what if the first star wars ended with the rebels blowing up some minor property of the empire, providing a barely noticeable inconvenience to the emperor?
rabban is a minor character (in the books especially). apart from serving as a disposable pawn in the baron's scheme to place feyd on the throne, and giving a backstory to gurney halleck, rabban really doesn't do shit.