What Thor3 should be about is a fiercely debated issue. Names have been loudly voiced such as the Enchantress; another one with much more justification is Balder.
Balder in the Norse Myths is the one God whose death brings about Ragnarok: the one event which has major consequences on the Nine Realms. If fans and Marvel Towers want Thor3 to visit the Twilight of the Gods then Balder it must be.
What we have seen so far is the rather happy blending of certified Mythology characters such as Bor interacting with comics inventions like Malekith (albeit Dark Elves are certainly myths-canon). Thor1 was offering the same gambit.
Ragnarok with Balder could be visited and give us the best Magician Asgard has to offer: namely... (no, not Loki) ... Karnilla. Though just like Feige altered the original MCU version of a Loki adopted very officially by Odin, we could have Karnilla as the Norn Queen raising Balder as her son.
Why this twist? As Thor2 ends up with an officially proper dead Loki and Thor gallivanting on Earth, Odin or the individual posing as Odin is facing the dilemma experienced by countless mortal rulers: the first duty of the king is to beget a heir to his dynasty! Odin with one dead second son, a dead wife and Thor having sent his resignation about becoming king must remarry and get himself a heir (and I am quite sure Loki has not seem this coming as some of his best laid plans can be glaringly missing the fine details).
Loki might be keen on finding himself a queen though I suspect the kind of ladies he will be presented will be boringly demure (here place one comical interlude); more importantly he learns Odin and Frigga have had a child together and said son/brother is raised by Karnilla. It transpires that during the Jotun wars which oversaw his own adoption, the Norn Queen was Odin's ally and for some mysterious reasons, little Balder was handed to her as a thank you. Karnilla is powerful, reeks of magic while physically she is just like in the comics: raven hair, green eyes and prone to self-questioning.
Loki meets Balder and feels unreasonably just as jealous of Balder as he was of Thor. Yes, we may venture a safe guess on who was Loki's mother by now.
Odin in the meanwhile is finally able to break up the spell of Odin sleep he has been submitted to and gets free. Thor3 sees Thor leaving Earth, chasing up Loki, coming to terms that not only he has a half brother but he is not Frigga's son. Balder is important as his existence allows the sorting out of all the loose threads Thor '1 and 2' left lying around. It also is staged mainly on Asgard and can introduce not only Karnilla but Hela!
Jane, Darcy and the male double act 'my name is Ian' and Stonehenge Streaker Selvig provide all the comic relief needed... if only by Jane meeting her real prospective mother-in-law Jord.
At the end, the two good boys Thor and Balder go after Loki and yes again Loki is about to die or rather to be handed to Thanos by Karnilla when she at long last realizes Loki is her real son and hands Balder to Thanos or to Hela.
In short Odin has it: Thor to rule over Midgard but not Asgard, Loki to rule over not only Jotunheim but Nornheim but wants Asgard, Balder happy to rule over Asgard but hostage to the Queen of the Kingdom of Helheim!
Thor 3 ends up with the accepted beginning of the Twilight of the Gods: Loki imprisoned because he is responsible for what happened to Balder and/or maybe a prisoner of Thanos and Odin is really angry (well more that in Thor 2), Balder is in the hands of the Goddess of Death.
Odin is now minus two sons out of three. Only Loki can free Balder from Hela and Loki is prisoner in the hands of the Other (which comes to a big surprise to Odin and Thor who realize albeit too late that Loki was from the events of Avengers 1 a thrall of the Mad Titan. It does not imply Loki is innocent but it mitigates a lot of his actions.
End of Thor 3: the cliffhanger. Will Thor find a way to free Loki to free Balder as if he does not succeed Ragnarok will happen! And you can add a prophecy as how the world will end or be saved depending on three brothers who are not brothers...
Avengers3 will see the mightiest of Earth Heroes fighting of Thanos, Surtur etc... and could give us the bittersweet but funny twist of Loki fighting on the same side of the Avengers... Expect some frustrated Hawkeye, some snarky RDJ and Fury to be shown what real power is...
As you can see if this editorial ends up in some far-fetched theories/AU/fanfiction, the original questions remain adequate regardless od how you answer to them...