Decrypting The Teaser
Tom Cruise as Iron-Man is right now the most hyped up MCU stuff. Hints from the Marvel creators themselves and teases in trailers and of course super logical fan theories reveal it all.
Multiverse Made It Possible
The rumors came in existence due to the introduction of 'Multiverse' and many hints in the trailer towards "An Iron Man" like the Legion of Ultron Bots and the existence of Illuminati & Professor X.
We'll discuss the hints and the teases later, but look at that, you can literally see a glowing Iron-Man with the 'Superior Iron-Man' suit, zoom into it & you'll also find Tony's iconic 'Beard' there.
The Superior Who?
The variant Tom Cruise is meant to play is called "The Superior Iron-Man" with a different silver armor and has three infinity stones, one on his head clearly, and does not wear any helmet.
The reason behind his glow is explained by Iron Man's Creator itself, that it's actually called Endo-Sym armor in the comics, and Tony built it when he went all evil in the comics and took over the world with it's virus disguised as an app.
Legion Of Ultrons? Say No More
We see high-tech shackles and legion of Ultron bots escorting Dr. Strange, created by Tony Stark. Ultron is an Infinity Stone upgrade to Jarvis that went corrupt. So, there could be a universe where Tony didn't let them go corrupt. Besides, you need a genius Tony Stark if you want to deal with multiverse threats.
The Existence Of Illuminati
Illuminati was actually formed by Tony Stark along with Professor X, Black Panther, Black Bolt, Namor The Sub-Mariner, and Mister Fantastic aka Reed Richards (Elastic Man)