When the third Captain America film was in the early stages of development, Robert Downey Jr. and Marvel Studios were still embroiled in discussions for a new contract to see the actor return as Tony Stark. So with one-half of the "Civil War" conflict potentially unavailable, the Russo brothers had one old comic arc in mind to replace it. Captain America #193 cover by Jack Kirby and John Romita
Speaking to Entertainment Weekly to mark the upcoming home release of Civil War, the brothers Russo revealed that, for a very brief time before Downey Jr's contract was sorted, the third Captain America film was going to be based on the 1976 comic book storyline "The Madbomb" (it can destroy the world!) by Jack Kirby:
Joe Russo: It was not a given that we were even going to do Civil War when we were talking about the next movie after Winter Soldier. So there was a period of time when we explored possibilities for Cap stories that did not include it. We spent a few weeks doing that, although Civil War came up fairly early in the process and once that happened it took over our brains and we ran hard at it.
Anthony Russo: 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.
The madbomb, as the name implies, was a bomb that used sonic waves to drive people into fits of violent insanity. That in and of itself would have been rather great to see unfold in a Marvel film, but honestly the bit that probably wouldn't have come along with it in the potential film is the best part of the arc.
The villains that deployed the madbomb were a group called the Elite, lead by one William Taurey. The Elite wanted to use madbombs to turn the US public into a ravenous horde that overthrew the government so the Elite could rule instead. But for Taurey, the beef with Cap was personal... because he was the ancestor of a British soldier killed in the Revolutionary War by none other than THE COLONIAL VERSION OF CAPTAIN AMERICA:
Oh, can you imagine a movie that would have included this plotline? It'd be hilarious. Alas, it was never meant to be -- Downey Jr. negotiated a new contract with Marvel to continue appearing as Iron Man, and we got Civil War instead. Probably for the best, really.