Iron Man (odds of dying: 5 to 1)
The trailers certainly want you to think that the end is near for Tony Stark. The whole MCU started with him in Iron Man in 2008 and it seems fitting that Stark would meet his own end as the 3-phase, 22-movie narrative comes to a close. There is also an arc from the first Avengers to consider.
Before there was a Civil War, Tony Stark and Steve Rogers were already at odds with each other. Steve accused Tony of being out for himself and that Tony would never lay down on a wire to let the next guy go over him. Tony’s response was that he would cut the wire, inadvertently reinforcing Steve’s own point that Tony might not be willing to sacrifice himself to save others.
Some believe that Tony will complete the arc that began in this exchange by sacrificing his own life in these next two movies. That is possible, perhaps even likely, but remember that Tony already proved to Steve and everyone else that we was willing to lay down on the wire when he took the nuclear missile through the portal in The Avengers. Tony had no idea he’d survive and even tried calling Pepper to say goodbye.
This is not to say that Tony will not die. There is a good chance he will, but the story does not require his death in order to be complete.
Expiring contracts are not much of an issue either, as Downey has proven over the years that he is happy to continue playing this character. Tony’s days of being the face of the MCU are probably numbered, but it’s not hard to see him as a supporting player with huge star power like he was in Spider-Man: Homecoming.
Captain America (odds of dying: 2 to 1)
Directors Joe and Anthony Russo have been working with a lot of Marvel characters, but they got their start in the MCU with Steve Rogers in Captain America: The Winter Soldier. Christopher Markus and Stephen McFeely, the screenwriters behind the next two Avengers films, have been with Cap for all three of his solo films. If one of the two biggest Avengers is going to die, Steve Rogers has a slight edge.
Ever since he dove on the grenade he didn’t know was a dummy at Camp Lehigh (and even before that), Steve Rogers has been ready to give his life to save everyone else. It feels like his destiny, even if it will take almost a century for it to be fulfilled. Steve Rogers is the ultimate hero of the MCU and making the final sacrificial play to save the universe is totally a “Cap move.”
Outside of the fictional story, there’s an expiring contract and Chris Evans’ back and forth attitude regarding his desire to keep going after Avengers 4. The comics have also provided a succession plan for the shield and title of Captain America, should Marvel Studios want to honor Steve Rogers’ legacy even without the character being alive. If Sam Wilson and/or Bucky Barnes are among the surviving Avengers, they could lead the Captain America franchise going forward.
Steve Rogers or Tony Stark will die in these movies. I’m just not completely sure which one and I’m certainly not ruling out a beautiful scene in which they both sacrifice themselves to save the universe. If it’s only one of them who dies, however, it will probably be Cap.