While the Ragnarok plot line is being heavily rumored for the next Thor movie, the story could also feature an appearance by one of the most famous villains Thor has with the help of his brother Loki.
One villain that has been rumored for the next installment is Surtur, a fire demon native to the extradimensional plane of Muspelheim land of the fire demons, one of the nine worlds in Norse mythology.
The character's first encounter with the Asgardian ruler Odin is told in flashback and establishes their enmity when Surtur is imprisoned by Odin inside the Earth after forming an alliance with the Trolls and trying to destroy the world in anger for Odin defying him, although he gives Odin a winged horse, hoping to be released one day.
In the comics, the seer Volla predicts that Loki will free Surtur and other enemies of Asgard and eventually bring about Ragnarok, a war that will end with the destruction of all the Norse Gods.
Surtur seems like he could be the most all-powerful character that Thor has faced yet as he was depicted as an immense and malevolent elemental fire demon in the comics whose power was of apocalyptic proportions. Surtur possessed strength and durability by far surpassing that of Thor; the ability to generate intense heat, flames or concussive force.
With the Ragnarok plot possibly happening, having Surtur and Loki teamed up against Thor seems like a scenario that could lead to a massive battle and large implications for the future of Asgard.