Assuming that Avengers: Secret Wars will be based on the comic of the same name, Captain America and Iron Man could return to the MCU via a brand-new version of Captain America: Civil War. Taking place on a patchwork world of realities following massive incursions and the end of Marvel's multiverse, 2015's Secret Wars took the "Battleworld" concept from the original 1984 version and made it something more compelling and far larger in scope. To that end, one of these regions was based on a reality where Steve Rogers and Tony Stark's war never ended, providing the perfect means for Chris Evans and Robert Downey Jr.'s versions to return for the culmination of the MCU's Multiverse Saga.
As seen in 2015's Secret Wars comic story line by Jonathan Hickman and Esad Ribić, the Illuminati were unable to prevent their primary Marvel Universe (designated Earth-616) from colliding with the Ultimate Universe (Earth-1610). Serving as the last in the long line of incursions that wiped out the multiverse, it was only thanks to the efforts of Doctor Doom that existence itself was salvaged. Forming Battleworld from a collection of surviving realities, Doom declared himself God Emperor. As such, one of these regions was known as the Warzone, effectively a version of America that had been ravaged by Captain America and Iron Man's Civil War in a reality where it lasted for far longer than in the original storyline.
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As seen in the 2015 Civil War comic tie-in series to Secret Wars from Charles Soule and Leinil Francis Yu, Captain America and Iron Man continued their war for years, eventually splitting the country in two as rivaling societies. Not only would this serve as a dynamic means to feature Captain America and Iron Man's return to the MCU, but it would also allow them to get things right by putting aside their difference to face a much larger threat (something they neglected to do when Thanos first came to Earth). Here's what happens in Secret Wars' version of Civil War, and how it could be incorporated into the MCU's version to facilitate an epic Captain America and Iron Man return.
In 2015's Civil War, the original event's final battle in the Negative Zone created a massive disaster, killing millions and creating a massive chasm that would be known as The Divide. On one side was The Blue run by Steve Rogers. Offering freedom, citizens of the Blue only had to follow two rules: harm no one and help when they could. Any wrongdoers would be handled by lawmen inspired by the Punisher. Likewise, Tony Stark controlled The Iron, a more civilized metropolis focused on law and order. Superhumans were closely monitored and trained by agents of SHIELD to keep their society safe and well. Due to the heavy losses accrued along with the deep pride and egos carried by both heroes turned full-time generals, this expanded Civil War lasted for years without end.
Considering the fact that Avengers: Secret Wars will be the end of the Multiverse Saga, it stands to reason that this culminating film will be a lot like the comics' version. This is supported by the fact that incursions have already been seen in Doctor Strange in the Multiverse of Madness as a significant future threat. Furthermore, the MCU Secret Wars will likely feature a massive assembly of heroes just like Avengers: Endgame, the final film in the Infinity Saga. While Iron Man gave his life to save the universe from Thanos and Captain America went back to his own time to reunite with his love Peggy Carter, the infinite possibilities of the multiverse necessitate their return in order to make Avengers: Secret Wars feel as big as it should.
It certainly seems as though adapting Secret Wars' Civil War tie-in series would provide the perfect way to organically bring back Captain America and Iron Man to the MCU. Featuring similar variants of Steve and Tony who never stopped fighting their war could be an exciting way to do just that without tainting the satisfying endings both heroes received from Avengers: Endgame ending. While Secret Wars' expanded version of Civil War is still focused on the original issues seen in the original 2006 storyline, the initial reason for why they started fighting ultimately becomes pointless after years of war, meaning that the MCU's Civil War plot could easily be used in a similar fashion. Regardless, Captain America's war with Iron Man could also be discovered by the New Avengers during the reveal of the MCU's version of Battleworld (right along with the realm's other regions that were once separate realities).
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While the concept of using an extended Civil War to bring back Rogers and Stark is exciting, the most compelling part would be the chance for a do-over, allowing both heroes to get the ending right this time. In the MCU, Civil War fractured the Avengers right before the events of Avengers: Infinity War, a division that didn't allow them to respond as best they could in the face of Thanos' invasion where he claimed the final Infinity Stones. While Steve and Tony would eventually join forces once more, it was only after half of all life was erased by Thanos.
Interestingly enough, it was discovered at the end of 2015's Civil War tie-in series that Skrulls had been manipulating the Blue and Iron to continue fighting without end since the beginning. While neither hero was fully blameless, Captain America and Iron Man immediately put aside their differences to take on the bigger threat, save their piece of Battleworld, and end the war in the process. If the MCU's Captain America and Iron Man were able to follow suit in this imagined version of the same event, it would certainly redeem their failings from the on-screen Civil War, making their theorized return in Avengers: Secret Wars even more dynamic and exciting.