First off I want to give a shoutout to @NovaCorpsFan because I remember reading an article on your theory for the behind the scenes plots of Thanos and that Loki is still working for him and his defeat by the hands of the Avengers were all part of his plan. With Loki back in Asgard he imprisons Odin and takes his place giving Thanos easy access into Asgard and the Tesseract and possibly the Infinity Gauntlet. This is a huge part of my theory and how Thor 3 will end so thank you for that and I loved the theory. Now on to how I theorize Thor 3 will end and how it will affect the entire MCU.
Now this is all more on how Thor 3 will end but I will give some thoughts on what will happen in Thor 3. I believe due to the events of Avengers:AOU. Thor returns to Asgard to find himself or to deal with the fallout of the past events. His return puts kinks into Loki's plan since he might find out he is not Odin. So he plans to distract Thor until he can finish bringing Thanos to Asgard to claim the stone and gauntlet. Now this could happen a number of ways be it the threat of Surtur or maybe the introduction of Enchantress and her ever loyal servant the Executioner.
In the end Thor is able to overcome whatever obstacle Loki throws into his way and discovers and frees his father Odin restoring the Allfather to his rightful place as king of Asgard. Yet before they can go and finally bring down the trickster god Loki he has finished bringing Thanos to Asgard to claim his prizes. Thor immediatly jumps in and challenges Thanos yet he is either quickly or after a shorttime of Thanos playing with him defeated by the much more powerful Thanos. Their mightiest champion standing no match for the Mad Titan the Allfather Odin himself stands before Thanos to challenge him. Wielding his all powerful Odinforce and spear Gugnir it seems for a moment Odin is able to fight back the Mad Titan yet in the end not even the Allfather is enough to defeat the Mad Titan and is defeated laid dying before his son Thor.
Thanos having defeated all those who could oppose him goes and claims what he sought all along. The Tesseract and the almight Infinity Gauntlet are his. Yet before he leaves he decides to give Thor and all of Asgard a parting gift be it through his own power or the stone from GOTG which he claimed before these events possibly in GOTG 2 he begins the destruction of all of Asgard. He leaves taking Loki with him leaving all of Asgard to their fates. Thor quickly goes and leads all the survivors he can through the Bifrost bridge be it to Earth or one of the other nine realms. Though he saves who he can there is no doubt that Asgard is gone and Ragnarok has come to be.
Now that I have gotten the events of Thor 3 out of the way this is how I feel this will affect the entire MCU going forward. First off having Thor and Odin going off against Thanos will be great for two reasons. 1. It shows off the power of Thanos and gives him a moments before Avengers Infinity War Part1 to show off his awesomeness. Seeing not only Thor easily defeated but in the end the Allfather Odin killed would be a great stakeraiser to show how great Thanos is. 2. It would be a great sendoff to Odin seeing Thor's father fight against Thanos would show off his power but would also be a great final moment for him to go out on.
Loki being with Thanos and in the end leaving with him as Asgard is being destroyed would set him up as Mephisto from the Infinity Gauntlet story. Being the good henchman that he is suppose to be yet in the end of the two part epic of Infinity War will be can finally have a change of heart and turn on Thanos possibly having a heroic sacrifice that can change the tide against the Mad Titan. Mephisto was a big part of the Infinity Gauntlet storyline and we don't have that character here and I don't see Marvel throwing in Satan any time soon so Loki can easily take that place.
Lastly with the destruction of Asgard we get the title of our movie Ragnarok the home of the gods has been destroyed their time is over. Their people descimated and without a home their place in the universe is forever changed. Yet this is not the only reason we also have a big familiar place in the MCU where we have been in two films now completely gone due to Thanos. This raises the stakes for the Avengers film so much because we see just how powerful and destructive Thanos is. It would be a great way to go into the Avengers Infinity War Part 1.
So there is my editorial on how I believe Thor: Ragnarok will end and how it will affect the MCU thank you for taking the time to read my article.
P.S. Forgive me for spelling and gramatical errors I am a terrible writer.