It's hard to imagine and, frankly, you probably don't even want to think about this -- but Robert Downey Jr. can't play Iron Man forever. In fact, we know he's not going to; RDJ underwent a very public contract extension negotiation in 2013, following the release and success of "Iron Man 3." At that time, the contract stated that he would appear in two more Marvel movies -- "Avengers 2" and "Avengers 3." The solo "Iron Man" franchise came to an apparent close at the end of the third film, and Downey doubled-down on there not being an "Iron Man 4" in late 2014.
But a few things have changed since Downey's 2013 contract extension. First, the actor signed up to play Tony Stark in "Captain America: Civil War," though it's a move Downey has made sound like an informal addition to his new contract. Second, Marvel announced that "Avengers 3" would actually be split into two films: 2018's "Avengers: Infinity War Part I" and 2019's "Infinity War Part II." We don't yet know if RDJ's contract includes both parts, but we can't deny that time has to be running out on this version Iron Man, so the question has to be asked: what happens when Downey is no longer the face behind Marvel's Iron Man mask?
As hard as it might be to imagine someone else sporting Tony Stark's fine-tuned goatee, it's probably easier for the higher-ups at Marvel and Disney to simply hire a new actor to chew up the Marvel scenery. After all, they've experimented with recasting roles already; Josh Dallas was replaced by Zachary Levi in the the "Thor" films' relatively minor role of Fandral, and even more noticeably -- and closer to the topic at hand -- Don Cheadle famously replaced Terence Howard as James Rhodes from the first "Iron Man" film to the second. And let's not forget the big one, when Marvel traded Mark Ruffalo for Edward Norton when they brought Bruce Banner into the Avengers fold. The success Marvel has had with Cheadle and Ruffalo compared to their predecessors may just embolden them just make the switch to someone else in roughly the same age range as RDJ and leave it unacknowledged in the story.
Or Marvel could decide to make Downey's recasting a whole thing and turn it into a major plot point. Could Helen Cho clone a "new body" for Stark, one that makes Tony look a few decades younger -- and possibly like Joseph Gordon-Levitt? Or, if Marvel really wants to reference a deep cut from Avengers canon, they could use the still unseen Time Stone to rewind Iron Man all the way back to his teenage years. No, that's not how the plot shook out in "The Crossing" but, well, the less said about that the better. Still, if Marvel wins audiences over with their first teenage superhero, Spider-Man, there's the slim possibility that they could look to repeat that success. Could teen Tony be the role Asa Butterfield was born to play?
The smoothest way to keep MCU continuity solid, keep an armored Avenger in action, and honor the invaluable role Robert Downey Jr. has had in making these films a success would be to simply give the Iron Mantle to James "War Machine" Rhodes. Cheadle's a proven Hollywood A-lister, and his role in the Marvel movie-verse is only becoming more essential; beyond appearing in two "Iron Man" sequels, Rhodey's been bumped up to Avenger status after playing a big role in "Avengers: Age of Ultron's" third act, with "Captain America: Civil War" on the horizon. Making him Iron Man would mirror the trajectory of his comic book counterpart, who operated as Iron Man for much of the early '80s. Rhodey is a great character that fans enjoy, and Cheadle brings so much charm to the role; giving him "Iron Man 4" seems like a solid bet.
There's also something to be said for sending the Iron Man armor off to the recycling plant when Downey leaves. Whether they kill Stark off -- imagine how big of a threat Thanos would seem if he killed Iron Man in "Infinity War Part 1"? -- or just have him retire to a secluded beach with Pepper Potts, it would be a reverent touch to permanently power down the red and gold suit. "Iron Man" is already a cohesive trilogy, and Cheadle's War Machine can operate as the Avengers' resident person-tank, no problem. It would also be smart for Marvel to build up Rhodes' War Machine brand instead of doing what so many comics do and have a black hero take on a role vacated by a white one. Marvel could let the "Iron Man" franchise rest while preparing "War Machine" for flight.
This is the longest shot of all, but if RDJ's contract carries him through both "Infinity War" movies, Marvel Studios could use that two-parter to clear the deck completely. After all, a lot of other Marvel actors are most likely approaching the end of their contracts, including franchise leads Chris Evans and Chris Hemsworth. Captain America does have two comic book heirs built into his franchise (Sebastian Stan's Winter Soldier or Anthony Mackie's Falcon), and Asgard is rich enough with characters (Loki, Sif, Heimdall, the Warriors Three) that Marvel could theoretically revisit it without Thor. But with so much retiring and replacing potentially on the horizon, Marvel could look at the surely epic and definitely cosmic "Infinity War" as a way to hit the reset button. Maybe the Reality Stone will warp a new team of Avengers into existence, or give the entire team slightly younger makeovers. Or perhaps it will end the universe altogether and 2019's "Inhumans" film will kick off a whole new era of an all-new, all-different MCU.