(Warning: The following contains potential — albeit entirely theoretical — SPOILERS for future Marvel movies, as well as definite ones for past comic books in Marvel's ill-fated 'Ultimate' line. Proceed with whatever level of caution that suggests to you is wise.)
Now, for all that Robert Downey Jr. is increasingly expected to sign on for one final Iron Man movie, there's also a general acceptance among most fans that the man who essentially is Tony Stark will eventually step away from the Marvel Cinematic Universe. Whether that's after the events of Avengers: Infinity War — Part 2 or the long-rumored Iron Man 4, then, is simply a matter of timing.
The big question that leaves us with, though?
Would The Iron Man Armor Retire With RDJ, Or Be Passed On To Another Actor?
Now, of course, there's always the possibility that Marvel will defy widespread expectation and recast the role of Tony Stark — heck, they could always bring back his teenage self, like the comic books inexplicably did that one time — but it seems far more likely that the Iron Man armor would be passed on to another hero, or retired completely.
Assuming that the complete absence of its most iconic movie hero (sorry, Cap) wouldn't be Marvel's first choice, it seems a little more likely that we would see another Marvel hero don the suit. The burning question, of course, being who. Don Cheadle's War Machine would seem the most obvious choice — seeing as he's taken over as Iron Man in the comic books before — but may be seen as a little old to act as a long-term replacement.
Is it possible, then, that we could see an entirely unexpected hero take over as Iron Man?
Could We See Natasha 'Black Widow' Romanoff Become The New Iron Man?
Or, perhaps more likely, could we see Scarlett Johansson's hero become the newly renamed "Iron Woman" — or, y'know, just stay Black Widow, only in some shiny new and decidedly kick-ass armor?
Now, strange a suggestion as that might seem, seeing as Stark and Romanoff don't especially get on, after that whole "secretly infiltrating Tony's company" thing...
...there's actually an intriguing comic-book precedent for the idea — one that goes back to Marvel's ill-fated "Ultimate" line of comic books, which just so happened to heavily inspire the MCU. Y'see...
Back In The Ultimate Universe, Tony Stark Made Black Widow Her Very Own Iron Man Armor
Now, in fairness, he did that as part of an elaborate scheme to propose (the two were dating at the time), and the whole plotline ultimately led to the big reveal that she was actually a traitor who'd been working against the "Ultimates" (that universe's take on the Avengers) the entire time, and had killed their butler, Jarvis...
...but right up until that treachery? Her suit sure did look awesome:
Now, of course, the odds of Black Widow turning out to be a dastardly double agent are about the same as Captain America turning out to have been in Hydra this whole time (wait, what?), but the idea of her donning a shiny black suit of Iron Man armor? Well, that might not be all that crazy at all. Y'see:
Giving Black Widow A Suit Of Armor Might Be The Perfect Way To Keep Scarlett Johansson In The MCU
Now, from Marvel's perspective, there would obviously be some sensibly pragmatic reasons behind asking Scarlett Johansson to take over the Iron Man armor (she's twenty years younger than RDJ, and is one of the few actors out there who's considered able to "open" a movie with her name alone). What's more, it would allow the company to fulfill its promise to give Black Widow her own solo movie, without actually having to take any kind of risk financially speaking (any movie with Iron Man armor in it seems sure to turn a profit at this point).
The logic could, however, extend both to Johansson herself, and to Black Widow as a character. For Johansson's part, there may well be some appeal to the idea of her role becoming less overtly physical — something that seems to be a factor in several actors, including Hugh "Wolverine" Jackman, pulling away from superhero cinema. For Black Widow as a character, meanwhile, it would make a good deal of narrative sense were she to see the adoption of a sleek suit of armor as both a notable power upgrade and a chance to redeem herself in the eyes of a global public that doesn't really trust her.
A kick-ass female Edward Snowden with Iron Man armor and a burning need to do the right thing?
That sounds like one hell of a modern Marvel movie to me...