Perhaps it will be a structure like this -- Hela is established as the main antagonist early in the film (like, when she crushes Mjolnir), Gamemaster is the antagonist in the Barbarian Planet section of the film (but that's the only place where he appears), and then when Thor and his allies attempt to retake Asgard from Hela in the third act Surtur is revealed as the film's true Big Bad.
I think I saw the film described as a "sci-fi/fantasy road movie," so I can see the film having different antagonists at different points in time in the film. That would, in some ways, mimic as classic comic book structure, with periodic cliffhangers and complete changes in narrative direction throughout the film.