Captain America a.k.a Steve Rogers was originally planned to enter Avengers: Infinity War towards the end of the film.
Directors Joe and Anthony Russo confirmed in the directors commentary of the film that one of the hardest decisions they had to make was where was the best place for Steve Rogers to enter the film. They wanted to make sure that Steve had an impactful introduction to the audience, the characters and the film. Steve Rogers was of course introduced in the film around 40 mins in when he and the rest of the “Secret Avengers” stormed the train station in Scotland to help Wanda and Vision. There is no doubt that the introduction of Steve behind the train was absolutely insane, however this was not always the intended plan.
Infinity War writer Steve McFeely confessed that there were drafts of the film that delayed the entrance of Captain America quite significantly. Christopher Markus was another writer on the film, he told us that Cap’s entrance into the film was when he tackles Corvus Glaive in an attempt to save Vision during the Battle Of Wakanda. The writers said that they had to start this film from scratch as they did not have much to model off of, therefore finding an introductory point for the star spangled hero was a bit of a conundrum.
While it most certainly would have been epic to see Cap enter the fray in a desperate attempt to save one of his own, it would have undoubtedly caused some conflict among fans due to one of the main players of the MCU having been introduced towards the very end of the film with not a lot to do, similar to the Luke Skywalker situation in The Force Awakens.
You can definitely see the difficulties that the directors and writers had to contend with for Steve Rogers in the film as he did not have that much screentime or dialogue in the film as it was which has already made some fans a little upset. So limiting his screentime even more would have only spawned more negative comments from fans as it does seem strange to have the leader of the Avengers have so little to do in the biggest event to ever materialise in the MCU.
While Infinity War may have taken away the focus from Captain America for the most part, there is no doubt that he will have a bigger role in the upcoming Avengers 4. Joe and Anthony Russo have already confirmed that Steve Rogers has a much bigger part to play in the next film and it will finally conclude his story arc that was begun in Captain America: The Winter Soldier, it’s safe to say that we will see Steve get a much bigger story focus in Avengers 4 which I think we can all agree is desperately required after Infinity War.