Captain America: Civil War was a pretty packed film, with two teams of battling superheroes and a couple of actual villains as well, but it could have been even bigger.
In the months leading up to the film, there were a lot of rumours going around that Red Hulk was going to be in it, and that Spider-Man would be receiving his Iron Spider costume. Both had some sort of basis, since Thunderbolt Ross was making a return after eight years out the MCU, while the Civil War comics sees Spidey get the costume from Stark. Ultimately neither came to pass, but both were close at one point, with Joe Russo telling ComicBook.com:
"We had so many characters in that movie as it was. We consider everything, obviously, but with so many characters in that movie as it was to try and entertain adding another character and to get to the Red Hulk, you have to add a back story. You have to substantiate the narrative to get to that. [Thaddeus Ross], on our list of priorities, he was there sort of as the government’s agenda and not to add another complicated super powered character to the mix."
On the subject of Iron Spider, meanwhile, he added: "Again, that falls in that category of anything's possible. We consider everything."
It's not a surprise to hear it was considered, but probably for the best that they didn't make their way in. Red Hulk is something you could try in a solo Hulk movie, but Civil War was a delicate balancing act, and it likely would've tipped the scales. As for Iron Spider, it would've worked had Spider-Man been in the film more, but this way leaves the door open for him to get it later down the line.
In terms of ways the film was almost different, there's also the fact it wasn't always certain they'd get to do Civil War, with that dependent on the presence of Robert Downey Jr. In the early planning stages, one idea they came up with if they couldn't get him was to adapt the story Madbomb, with Anthony Russo telling EW:
"There was a period where we did discuss a third act that revolved around the Madbomb from Cap mythology. It didn’t have anything to do with Civil War, and if we couldn’t get Downey – in the very, very early conversations before we nailed him – somebody pitched the idea of a third-act that revolved around the Madbomb, which makes people crazy. It almost like zombifies them – but not literally."
It sounds like an interesting route to go down, but obviously they did get RDJ in the end, and the rest is one of the very best entries in the MCU.