The directors of Avengers: Endgame rule out the possibility of alternate versions of Captain America and Iron Man appearing in future movies. One of the foundational and most important parts of the Marvel Cinematic Universe is the ongoing nature of its story. As difficult as it may be, Marvel Studios has tried to create and maintain a reasonably consistent timeline. After twenty-one films of build-up, Anthony and Joe Russo crafted Avengers: Endgame to wrap up everything that came before.
The movie served as the finale of the Infinity Saga, even if Spider-Man: Far From Home is technically the last film in Phase 3. Saying goodbye to Thanos and the Infinity Stones in the present day are big reasons why, but more so because of the heroes that audiences had to say goodbye to. From here on out, the MCU will be without Steve Rogers' Captain America (Chris Evans), Iron Man (Robert Downey Jr.), and to a lesser degree Black Widow (Scarlett Johansson), since she has a solo movie on the way. But, with alternate timelines and the multiverse at hand, could any of them return?
If the Russo brothers are to be believed, then that will not happen. During their panel at San Diego Comic-Con 2019, they were asked if alternate versions of Captain America and Iron Man could appear in future films, such as the Secret Wars movie they keep saying is the one film that would bring them back to the MCU. Joe Russo adamantly responded with, "No, not possible."
Even though the Russo brothers are not currently involved with the future of the MCU, the belief that alternate versions of Captain America and Iron Man are off the table is probably a sentiment that rings true for everyone at Marvel Studios. The death of Iron Man and Steve finally getting his dance with Peggy were perfect and emotional send-offs for these characters, so bringing them back as alternate versions of the characters when Earth is in a bind would go back on this. Instead, if Evans or Downey ever do reprise their MCU roles down the road, expect it to be either in flashbacks or possibly as old Steve, in Evans' case.
That said, the Russo brothers weren't against bringing back characters in other ways during Avengers: Endgame. Aside from the multiple cameos that came from the time travel plot, both Thanos and Gamora returned to the present-day despite previously being killed. They may not be from an alternate timeline with different experiences, but the decision to bring them back into the story did create a new timeline.
So, if that method of bringing back characters is acceptable for the Russos and Marvel Studios, we can't quite count out the possibility of future heroes going back in time to get Captain America, Iron Man, or any other character. And, with the advancements in de-aging technology, there is almost no limit to how and who they could bring back. Hopefully, the MCU won't go back to the time travel realm again anytime soon, now that Avengers: Endgame has done so - and proven just how complicated it can be.