Endgame!Steve is not the real Steve: sometime before Tony comes back to Earth, Steve is kidnapped and replaced by an evil alt! version of himself (maybe it's just a Skrull, maybe it's an alien, author's choice). Natasha notices something off about him: author's choice whether she ever finds definitive proof that he's an imposter. She confides her suspicions to Bruce and/or Rhodey, but none of them are able to do anything.
Everything else in the movie proceeds as it happens, including the fight with 2012!Steve, except that in the final battle against Thanos alt!Steve cannot lift the hammer.
After the battle, Bucky immediately seeks out Steve, whom he has been in an romantic relationship with when he was dusted (author's choice as to when this relationship started, Bucky's sexuality, etc.). Alt!Steve is practically indifferent to his return. All he says is "Good to have you back" and locks himself in his room. The next day, Bucky sees alt!Steve staring wistfully at the Compass, and alt!Steve starts droning on about how much he misses Peggy. Bucky is heartbroken and angry and downright confused: he's convinced that there's something up, but at the same time starts to think that everything he ever had with Steve was all in his head. He takes comfort in an equally confused Sam (not romantic, just moral support).
Alt!Steve goes to return the Stones and comes back an old man. Author's choice whether or not he went to an alternate reality or stayed in the same one. Bucky doesn't want to talk to him. Soon after, Bruce and/or Rhodey confide in Sam and Bucky about the suspicions that Natasha had voiced. Bruce in particular is dumbfounded that Steve would so carelessly manipulate time like that. Maybe Wanda visits old Steve and senses something off about him (but he's telepathy immune so she can't just rip the answers from his head). So Sam, Bucky, Wanda, Bruce, Rhodey, and whoever else the author wants all team up to solve the mystery of alt!Steve.
They end up not only having to fight old Steve but having to go back in time and retrace his footsteps (specific reasons up to the author). They go back and question 2012!Steve about his interactions with alt!Steve, maybe recruit him to the cause. They go back to the 1950s when he married Peggy, where they discover that alt!Steve essentially forced her to marry him (I'm thinking maybe he threatened Daniel Sousa when Peggy first refused his proposal (because of course she would), but it's the author's choice how dark this gets).
Eventually, the remaining Avengers, 2012!Steve, and 1950s Peggy team up against alt!Steve and defeat him once and for all, and get the real Steve back. I have a personal preference for the real Steve having been deserumed by alt!Steve, but author's choice again.
I'd be willing to discuss beta-ing this fic if anyone wants to write it!