Really, a Thor movie with Surtur as the main antagonist is the perfect vehicle for MOARLOKI. Surtur is the driving force of the plot and technically the main antagonist, but he's more like a walking thinking natural disaster than a villain who's a character with motivations and a personality, and the real character conflict comes from Loki and Thor butting heads and sorting out their **** while they both attempt to avert this crisis in their own ways. You don't need to spend any time on developing Surtur, because he's really just a big scary thing that's big and scary because our feeble minds cannot comprehend how massive and silly he is, so all of the character stuff can focus on the two brothers.