At the end of Thor: The Dark World we see that Loki has not only survived the battle on Malekith, but has somehow replaced Odin on the throne of Asgard. It would also seem that Odin has somehow died according to Anthony Hopkins and Alan Taylor. I say that this ending absolutely leads to a third Thor movie where Loki is once again the main threat. To keep this from being a repeat of the first Thor movie, however, I have come up with a movie that really ups the ante.
The movie would at first seem loosely based on the first Loki miniseries where he is ruling Asgard with an iron hand. The difference is he has not defeated Thor because the God of Thunder is living on earth with Jane and fighting with the Avengers. To keep Thor from finding out his treachery Loki has killed Heimdall, and replaced him with someone loyal to him who lets no one pass out of Asgard into the other Nine Realms except Loki. However, it is plaguing Loki that he has only accomplished half his goal with Odin's death, and that his brother still lives and may yet end his rule.
Then Loki somehow finds about an ancient prophesy in which he is supposed to bring about Ragnarok, the end of all things. Thinking this to be the perfect way to finally be rid of his brother, and as a way to rebuild an Asgard and a Midgard in his own image, Loki happily accepts this. So he decides to team up with three of Asgard's most dangerous foes:
1. Surtur
This is an enemy Loki has teamed up with in the past to bring about Ragnarok. Also Thor fans would already be familiar with him from the video game Thor: God of Thunder. Also this is the enemy that many fans like myself will somehow appear in the last Thor movie. Why not in this way.
2. Mangog
This a team-up that would make its way into the movie from the Thor video game. Also this being is pure rage making him stronger than the Hulk even. It could certainly give Thor a run for his money.
3. Midgard Serpent
The midgard serpent is an enemy that has recently made his way out of the comics and into the TV show Avengers Assemble after being controlled by Doctor Doom. What many may not know is that the Midgard Serpent is a child of Loki in the comics. This could easily lead to a flashback of Loki's early life. Also this serpent is known for severely wounding Thor in the Fear Itself story.
With his team assembled Loki would attack Asgard, but not before wreaking havoc on earth to reveal to Thor and Jane that he is alive and that he is going to destroy Asgard. Loki then challenges Thor to try and stop him upon which a short fight ensues leading to the death of Jane and Darcy which prompts Thor to return to Asgard.
However, Thor sees he cannot do this alone and decides to team up with an Avenger or two specifically Hulk or Iron Man since Cap, Hawkeye, and Black Widow here. With the heroes returning to Asgard to prevent Ragnarok the most epic battle since The Avengers ensues. In the end the monsters are defeated and for his sins Thor kills Loki ending his threat once and for all. Thor then sees he has to remain on Asgard as it's king in order to rebuild the kingdom. In a post-credits scene Iron Man and/or Hulk would return to Earth where they see the world is under attack by Thanos and the Chitauri.