5. Bor
Odin's father was first introduced in Thor: The Dark World's opening sequence, and while Bor was victorious over Malekith that day, it has since been revealed that he later fell in another battle. In the comic books, a time travelling Loki is revealed to be responsible for his demise, turning Bor into snow in front of Odin years before he would adopt the trickster. This was all part of Loki's plan, as he would later undo his spell and resurrect Bor in the present day. However, he brainwashed him, and Thor went into battle with his grandfather not knowing who he was fighting. As Bor's powers reached a point where it could wipe out the whole planet, Thor had no choice other than to deliver a fatal blow to the Asgardian. When it was revealed who he had killed, a grief stricken Thor was banished from Asgard. If it isn't already obvious, this would make for a great movie, and it gives Marvel the perfect excuse to bring back this fairly obscure character from Thor: The Dark World. An appearance in Thor: Ragnarok seems unlikely, but tackling this story in Thor 4 would be a smart move by Marvel.