I don't know about Thanos, but I think Surtur is going to be a big part of this movie actually, without even appearing on screen. Even if we don't see him, I'd bet that he'll be name dropped multiple times, then a possible credit scene. Look at Odin's narration though. Odin is wise, Loki may have experienced the darkness. But Odin KNOWS that before the universe, there was darkness, how? What if Surtur is trapped? Odin was the one who trapped him. Given the two characters history in the comics, if Surtur is indeed trapped, it wouldn't be far fetched to guess, and well, flat out assume Odin is the one who trapped him. Perhaps long ago, maybe that's what the dark elves and asgardians previous fallout was about. If Odin trapped Surtur somehow, then he would most definitely be the only one to know about it. Malekith being the big bad of the movie, I almost see him essentially worshiping Surtur, or perhaps the dark elves worship him as the god he is, Malekith is a follower of his?