The plot for "Thor 3" is still up for speculation but we do know two things from where "Thor: The Dark World" left off and the fact that we are already on Phase 3 of the shared Marvel Cinematic Universe.

All of the movies that Marvel has lined up for Phase 3 seems to be involved with the Infinity Stones. We know that one of them is in Asgard: The Tesseract that the Avengers recovered. There were some viewers who questioned why they entrusted Asgard with the Tesseract knowing that Loki is there and he actually started this whole thing.

Nevertheless, it is already a fact that an Infinity Stone resides in Asgard. But how does this relate to Thor 'betraying' Asgard? With Loki at the helm and no one else suspecting (so far), Loki (as Odin) can paint any picture he wants with regards to his brother.

Other speculation from God of Mischief:

"Comic Loki triggered Ragnarok by leading an army against Asgard. MCU Loki already has the seat of power, so what would compel him to sit upon that throne, stroke his thumb across his chin and think, 'hmm. I know, let's just.... destroy everything! End it all!'"

Loki can convince Asgard that Earth (Midgard) has stolen the Tesseract to use it against them and that Thor is fighting on the other side. That would rationalize an Asgard attack on Earth!

However, things might not go according to Loki's plan. From God of Mischief:

"Well (and bear in mind this is just me speculating here) perhaps someone will twig that it's Loki on Asgard's throne rather than Odin, and overthrow him. This could see him escaping, and incite him into seeking out the enemies of Asgard (cue Surtur, the fire demon of Muspelheim) to commit the ultimate vengeance against the Realm Eternal."

A war of this extent between the various realms would result in Ragnarok. It would be similar to the comic book Ragnarok but it has to relate to the what's happening with the entire Marvel Cinematic Universe-which is the chase for the Infinity Stones.

As always, this is speculation and rumors. Thor 3 is 'in the works' and its release date would probably be late 2016 or 2017.