Wait, Has Iron Man Been Powered By The Tesseract Since 2010?
Is Iron Man powered by the Tesseract? In Iron Man 2, Tony Stark develops a still-unnamed new element to replace the toxic palladium in his arc reactor that's slowly killing him, giving him immense new energy and starting him on a track to creating the House Party, the Iron Legion, Ultron and many more high-tech suits. This element comes from a theory developed by Howard Stark and, in the movie's novelization, was described as "Vibranium", but the metal's presence in Captain America: The First Avenger, Avengers: Age of Ultron and Black Panther renders that non-canon.
Indeed, the sequence in Iron Man 2 where Tony synthesizes the new element has connections to a different area of the MCU. The equipment he uses comes from boxes marked Project Pegasus, the group analyzing the Space Stone in The Avengers and later revealed to have been active since at least the 1980s in Captain Marvel, with involvement from Dr. Wendy Lawson and, presumably, Howard Stark. With a drawing of the cosmic cube breifly shown, Iron Man 2 was the first introduction of the Tesseract in the MCU, and it could yet prove to be one of the most important.
Based on what's been revealed about Project Pegasus, it would seem that his new element derives from research done around the Tesseract; the theory from Howard Stark and the enactment by his son using tech from Lawson's team. At the least, this means that Iron Man's arc reactor from Iron Man 2 onwards (assuming the tech in the device is the same after it stops being used to keep the shrapnel from his heart in Iron Man 3) is linked somewhat to Space Stone technology.
For the longest time, the assumption would be that it simply unlocked knowledge of a new molecular composition. But, now Captain Marvel has shown that Mar-Vell was able to create replicas of Tesseract energy, it's possible to go a step further. Was what Tony made actually a replica of Tesseract techology?
This would mean Iron Man is considerably more powerful than many assume, and has been since before the Avengers first assembled. While that doesn't reframe any of his missions, it would provide a neat explanation for how his tech after just a few years of development can hold its own with beings who are basically gods. It would also line up similar Infinity Stone derivation in Avengers: Age of Ultron, where the genocidal robot came from the Mind Stone's "brain".
With that sequel aspect in mind, it may be that even Tony isn't aware of the implications of what he's made. Project Pegasus and its purpose was a massive secret (and Howard Stark loved keeping those from his son) and while he did uncover S.H.I.E.L.D.'s plans to use the Tesseract for their own means in The Avengers, but that was only ever to wield power and never connected back to the cube.
As already mentioned, Tony Stark wouldn't be the first hero in the MCU timeline to be powered by the Space Stone; Captain Marvel revealed that Carol Danvers' gained her superpowers when exposed to Mar-Vell's hyperspeed engine powered by the Tesseract. This works nicely from a more abstract point of view. Carol has long been likened as a Captain America or Thor replacement due to her military background and period origin story or cosmic power respectively, but this aspect ties her more closely to Iron Man: she is pretty much a distillation of aspects of all three core Avengers.
But beyond potential symbolism, this could have some narrative importance. Considering how Captain Marvel's immense power-set is expected to be essential in stopping Thanos in Avengers: Endgame, could a developed connection between Iron Man and the Space Stone also play a part? Or will this prove to be the missing piece in how Tony Stark and Nebula return to Earth at the start of the movie?
Admittedly, this is just a theory and there's not much evidence beyond Iron Man 2 to back up this thinking. Conversely, though, there's little to work directly against it. After all, while the movie in question is nine years old, it was full of key setup for the future of the MCU, with a S.H.I.E.L.D. map that introduced Wakanda and Atlantis, as well as a retconned cameo from Peter Parker. This Tesseract arc reactor, however, would take things to the next level.