Thor: Ragnarok promises to be a welcome return for Thor and the Hulk, both of whom were deliberately kept away from the implosion of the Avengers during Captain America: Civil War. (For a moment, Chris Hemsworth even thought he was fired when he didn't receive an invitation to Civil War). Just going on the delights we've been treated to in the first Thor: Ragnarok teaser, it looks like being isolated from the rest of the squabbling Avengers was the best thing for the Odinson and the Green Goliath.
In truth, Marvel Studios was wise not to involve Thor and Hulk in Civil War. That story was decidedly Earthbound and steeped in geo-political issues, involving the signing of the Sokovia Accords as well as the personal history of Tony Stark and the secret murder of his parents by Bucky Barnes, the Winter Soldier. Hulk and Thor both fought Ultron in the pivotal Battle of Sokovia, but their presence and power levels - both are classified as "a couple of nukes" by Secretary of State Thaddeus Ross - would have drastically escalated the already costly conflict between Team Cap and Team Iron Man. The two-year gap between seeing Thor and Banner again also puts distance between one of the more unpopular subplots that occurred in Age of Ultron: the budding romance between Bruce Banner and Natasha Romanoff.
However Thor's outer space "buddy cop-road trip" adventure with Hulk to eventually save Asgard from destruction turns out, eventually all Rainbow Bridges must lead to Avengers: Infinity War, and the six most prized objects in the MCU, the Infinity Stones. While most of his previous adventures in the Marvel Cinematic Universe kept him bound to Earth, either working with his now-former love interest Jane Foster (Natalie Portman) or with his superhero allies the Avengers, Thor parted with his human friends at the conclusion of Avengers: Age of Ultron to pursue answers about the Infinity Stones.
When he received a vision during Age of Ultron, Thor became aware that several of the Infinity Stones have appeared on Earth. Thor has personally encountered three Infinity Stones already: the Space Stone (the Tesseract), now housed in Asgard; the Mind Stone, now in the Vision's forehead; and the Reality Stone (the Aether), which was housed by the Collector in Knowhere but hasn't been mentioned since his museum was destroyed. Thor may be aware that the Time Stone is in the possession of Dr. Stephen Strange, in the form of the Eye of Agamotto, after they conversed in the end credits scene of Doctor Strange. Thor is likely not yet aware of the Guardians of the Galaxy's encounter with the Power Stone (the Orb), which is now locked away on Nova Prime.
This leaves the sixth and final Infinity Stone, the Soul Stone. It's whereabouts are currently unknown, but there's plenty of evidence to suggest it's also on Asgard, hidden in plain sight. Quite possibly, the Soul Stone has always been in the possession of Heimdall (Idris Elba), the gatekeeper of Asgard. Heimdall has boasted, "I can see the Nine Realms and ten trillion souls." His eyes are orange, which is keeping with the color scheme established for the Soul Stone, and when he came to Thor in the dream Scarlet Witch hexed Thor with in Age of Ultron, Heimdall was blind and raving that Thor would be the death of Asgard. It could be the Soul Stone that Hela uses to destroy Asgard in Ragnarok.
We'll have to wait until Ragnarok to find out if this theory proves correct and the answer to the final missing Infinity Stone will at long last be revealed. However, Ragnarok will be the last MCU film dealing with intergalactic affairs before Infinity War arrives in theaters on May 4, 2018. The only other MCU film between Ragnarok and Infinity War will be Black Panther on February 18, 2018, but with that film set to be about the origins of the Black Panther and resolving the subplots of Steve Rogers hiding out in Wakanda, it's less likely Black Panther will directly involve the Infinity Stones.
Where Ragnarok ultimately leaves Thor and Hulk remains to be seen, but one thing is certain: they are re-assembling with their fellow Avengers in Infinity War somehow. There are rumors the Guardians of the Galaxy may appear in Ragnarok's end-credits scene, so perhaps Peter Quill (Chris Pratt) will offer the Milano as transport back to Earth to Thor and Hulk, depending on what path Quill's own journey to meet his father Ego (Kurt Russell) in Vol. 2 sets him on. Or perhaps Thor and Hulk never quite make it back to Earth. After all, four of the six Infinity Stones are not on Earth, anyway.
The driving force of Infinity War is going to be Thanos (Josh Brolin), who, after trusting in several failed intermediaries, finally resolved to take a DIY approach and personally gather the Stones for his Infinity Gauntlet. With only two Infinity Stones on Earth, one possessed by an Avenger and the other possessed by the Sorcerer Supreme who is expected to be an Avenger one day, Thanos is bound to drop by Earth to come collect. Thanos paying Earth a visit should be the catalyst to bring Earth's superheroes into this grand conflict to prevent universal armageddon. The real action will be in outer space, and with Thor, Hulk, and the Guardians already out there in the void, why have them return to Earth any time soon?
Along with his experience with the Space Stone, Thor is extra vital in pursuing the missing Reality Stone/the Aether, which hasn't been seen since the Asgardians gave it to the Collector (Benicio Del Toro) in the end credits scene of Thor: The Dark World. Thor is the only one who has experience dealing with the Reality Stone, just as the Guardians are the ones who know the dangers of the Power Stone. A quid pro quo arrangement between the Guardians and the Odinson is likely.
But beyond the epic displays of superpowers, the cosmic conflagration against the ultimate evil of Thanos, and the potential for Marvel Team Ups in Infinity War, what fans really can't wait for will be the personal interactions between the heroes. It's been years since Thor has seen Tony Stark and Steve Rogers; it will also have been years since Steve and Tony last saw each other. The truly memorable fireworks that will erupt won't be because of repulsor rays or a hurled vibranium shield, it'll be when their strong personalities clash. At the very least, Tony will have a wisecrack about Thor's new haircut. And what will happen when Banner and Natasha see each other again, after he ended their budding romance by disappearing off the grid at the end of Age of Ultron? Let's not forget the fun we're expecting when Earth's Mightest Heroes, including a teenage web-slinger, meet a talking raccoon, a baby tree, and a motley crew of blue and green alien misfits. By the conclusion of Infinity War, all Marvel fans can delight that every single one of those heroes will be able to call each other "friends from work."