This is STRICTLY an opinion piece on the plot I would like to see.
Thor 3 would open with a story, told by Loki, in the guise of Odin, the All-Father...
A FLASHBACK to MUSPELHEIM, the realm of the Fire Demons and their lord...
A young and arrogant Odin, along with his brother's Vili and Ve stand defiant before him.
The daring brothers do just that, and steal the Eternal Flame. But not without Surtur hellbent on their destruction and the reclamation of the flame. Knowing the King of the Fire Demons will hunt them through the nine worlds, Vili and Ve commit a courageous act of valor.
His story told, Odin/Loki retreats to the dark recesses of Asgard.
Somewhere in the deepest reaches of the cosmos the resounding BANGS of a hammer build, as a cosmic figure works hard.
THOR, upon returning home to investigate the recent visions regarding the future, and the infinity stones, finds his father Odin (Loki in disguise) to be sullen, morose and secretive about just what the visions may mean. With the help of his kinsmen, the Warriors Three and Lady Sif, Thor unravels a plot to seize the throne of Asgard and more the plundering of the Odin's treasure vault, which contained the Tesseract, the Infinity Gauntlet, the Eternal Flame, The Casket of Anciet Winters and the Eye of Agamotto.
With the plan revealed, it does not take long for the culpit to reveal him himself and his machinations.
With Loki in power, all Thor and his cohorts can do is bide their time and wait for their opening.
Seizing the moment, Thor rallies his closest allies and attempts to overthrown his stepbrother. But seeing that they are doing harm to their fellow Asgardians who remain loyal to the throne, if not the ruler, Thor, the Warriors Three, and Sif cease their insurgency and are taken into custody.
Unbeknownst to them all, the greatest threat to their existence has has returned.
The forever long ago seal that Odin placed on Surtur is destroyed.
Fearing the destruction that Surtur will surely bring, Loki attempts to flee Asgard. Attacking and overpowering Heimdall he uses the Rainbow Bridge to retreat to Midgard/Earth. His act of cowardice leaves open the gate for Surtur's demon forces to invade Earth.
With Loki abdicated, Thor rally the troops. He tasks the Sif and the Warriors Three with defending Midgard, while he takes the battle to Surtur.
On earth, the recently arrived Loki realizes that not only Asgard, but Midgard will be doomed as well.
While Loki capitulates, the battle between the Asgardians and the Fire Demons wages on.
Though they fight valiantly, the fire demon horde begins to overwhelm Sif and her forces.
But reinforcements arrive when all seems lost.
The two forces Asgardians and Avengers come together...
While the renewed battle rages, Thor's mortal allies, Jane Foster, Dr. Selvig and Darcy attempt a plan to use one of the vault room weapons to their advantage.
Back on Asgard, Surtur presses the advantage. He handily defeats an already weakened Heimdall.
Thor, seemingly defeated, Sif senses that things are not well in Asgard.
With Thor incapacitated, Surtur makes his approach on Asgard and the Eternal Flame.
On Earth, the combined might of the Avengers and the Asgardians overcomes the Fire Demons.
While Jane, Selvig and Darcy are able to put their plan into action.
Desperate, both Thor and Loki dig out chunks of ground to free their father. But to no avail. Odin is lost.
And just as his father before him, the ODIN-FORCE strikes Thor and levels the area. When Thor comes to, Loki is no where to be seen. And Asgard has a new King.
Thor 3: Ragnarok closes with Thor on the throne of Asgard.