"Thor 3" has no release date announcements as yet. But Thor has a lot of plot points isolated in Asgard especially after the last scene in "Thor: The Dark World."
With Loki at the throne of Asgard, anything could happen and the God of Mischief can ally himself with the other nine realms. He did that with his father, Laufey the Frost Giant.
The identity of Loki's mother was never discussed. One of the many oddities of Loki is his stature that is not at all close to being a frost giant. Also, his magic abilities are also a mystery. According to Unleash the Fanboy, his lineage is more complicated than we think:
"It was pointed out during the interaction between Frigga and Loki that Loki has no idea who his real mother is. We do know that his father is Jotunhiem's King Laufey. My theory is that his mother wasn't from Jotunhiem (I'm sure there is also an explanation in the comics as well). In the comics, Hela is actually the daughter of Loki along with a few other mythical creatures. I wouldn't put it past Marvel to switch that up a bit for the movies turning her into his biological mother instead. It would explain his size along with his expert use of magic. This could lead to the introduction of the Valkyrie (Brunnhilde), the realms of the dead (Hel and Valhalla) and Mephisto (Marvel's version of the devil)."
It is also believed that Loki knows this, or that he has an inkling that his mother is neither Asgardian nor a Frost Giant. It should all be resolved at the Third Installment.
This is another possible plot which is different from the Ragnarok theory.
Speaking of theories, there is no explanation on what happened to Odin. It is unlikely that he agreed to Loki's plan. Did Loki kill his own adoptive father?
From Slashfilm, Kevin Feige was vague:
"Clearly I like the notion that some people think he might die in this movie. That works to our advantage. I wouldn't spoil that necessarily, but I do like that afterwards people can, and there will be references... There are no specific plans, but if in specific movies somebody references Odin, that means that something else is going on up there, which I think is cool."
Killing an Asgardian god, especially their king, is easier said than done. There will be more clues as we go along though.