In one of the more enticing scenes of the Avengers: Infinity War trailer, we seen the return of the Hulkbuster Armor from Avengers: Age of Ultron. From various scenes in the trailer, it's becomes clear that the Hulkbuster armor is deployed in Wakanda; however, the person piloting the armor may not be who fans think!
From what we've seen in the various Infinity War trailers, Tony Stark is not the person in the Hulkbuster armor. While Tony starts out in New York City at the beginning of the film, he clearly ends up off-world, in the company of Gamora and Nebula, battling Thanos on the plane of Battleworld. There's no real hint that Tony is ever part of the Wakandan defense effort; instead, the Hulkbuster armor's new pilot is the most ironic choice in the bunch.
If you look closely during the trailer you can see the foreshadow: Bruce Banner working on the Hulkbuster arm in Wakanda, when he seemingly first reconnects with Black Widow. It seems that in an effort to avoid Hulking out and possibly losing himself again, Banner finds another way to join the fight with his fellow Avengers; Banner, not the Hulk.
This is more than just a theory, really. Now that the Infinity War trailer is out, some recently-leaked LEGO set descriptions for the film are all-too-accurate in their SPOILERY descriptions. In regards to this Hulkbuster sequence, here's a small snippet of what the LEGO set describes: "Danger! Proxima Midnight captured the shooting tower and shoots on Hulkbuster Bruce Banner!" It seems that Thanos' top warrior Proxima Midnight storms the Wakandan defenses, forcing Banner to suit up in the Hulkbuster armor to stop her. There are a lot more SPOILER-filled details to this sequence, but you'll have to read those HERE.
Avengers: Infinity War opens in theaters on May 4, 2018. Thor: Ragnarok is now playing in theaters. Other upcoming Marvel Cinematic Universe movies include; Black Panther on February 16, 2018, Ant-Man and the Wasp on July 6, 2018, Captain Marvel on March 8, 2019, the fourth Avengers movie on May 3, 2019, the sequel to Spider-Man: Homecoming on July 5, 2019, and Guardians of the Galaxy Vol. 3 in 2020.