In the 1960's, Joe Simon sued Marvel for ownership of the character. Marvel asked Jack Kirby to create two new Captain America designs, with the idea being that if Marvel lost the rights to the original, they could quickly introduce a new version of the character. Simon ended up settling, and Kirby later recycled one of the new Cap designs for Captain Glory.
Captain America #300 by J.M. DeMatteis would have been a double-sized issue featuring Captain America becoming a pacifist, being rejected by superheroes, the government, and the world, then assassinated by Nomad and replaced by The Falcon or Black Crow.
Captain America vol. 3 #14 by Mark Waid would have shown more of the Red Skull's twisted perspective, including the Red Skull's origins with Hitler, and his viewing Captain America as a Hitler figure that has to be killed.
DeMatteis was originally hired to write an oversized treasury edition one-shot to tie into the Captain America TV movie that was released in the 70s, but the one-shot was cancelled. The contents were later published in Captain America #261-263 as the "Celluloid Heroes" storyline.
Waid had planned to use Kang as a major villain in his run, but due to Kurt Busiek using the villain in The Avengers, he ended up compromising by having Michael Korvac pretend to be Kang. Ironically, this was essentially recycling an idea Waid had planned when he was writing Ka-Zar, where he'd wanted to have Korvac impersonate Thanos. When someone else called dibs on Korvac, he scrapped that plot point and used the real Thanos instead.
Captain America dies in Dan Jurgens' Captain America vol. 3 #50, which would have been addressed by a follow up mini-series by John Ney Rieber, who had planned to write the first story arc for Captain America vol. 4, which ended up being scrapped and modified due to the World Trade Center attacks, ending up as volume 4's second story arc.
Before the real return of Bucky Barnes, there were plans for a story called "Bucky Returns", which would have revealed that Bucky had survived all of those years ago, but was now dying of old age. Steve would have went to Bucky's side, finding out that he was a quadruple amputee and disfigured, leading to a Tear Jerker-worthy scene where the two would reunite just before Bucky passed on.
Speaking of Bucky, Bucky was supposed to have returned the shield to Steve after he came back in Captain America: Reborn, but fans grew to love the Bucky Cap, so he got to stay longer.
Mark Waid had wanted to do a story rationalizing the absurdity of Bucky's secret identity (or lack thereof), but never got the chance. The story would have revealed that the Intelligence Office had fabricated a backstory for the boy, explaining why nobody thought "Bucky" was a risky choice for a superhero alias despite it being his real name.
After the end of Captain America and The Falcon, there were plans for a solo series starring The Falcon. Plans fell through when Christopher Priest decided he didn't want to be pigeonholed as a writer who only worked on "black" comics.
Captain America and The Falcon was also supposed to feature a romance between Cap and Scarlet Witch as one of the main hooks for the book's second year. The romance plot had to be moved up and drastically shortened due to the events of Avengers Disassembled taking Scarlet Witch off the board for the next several years.
There were plans for a major arc which would have seen Falcon inadvertently causing Cap's death. The plan was that this would lead into a Bat Family Crossover between several books, each showing how the Marvel Universe was dealing with the aftermath of this event. Ed Brubaker's Captain America book would feature several potential Legacy Characters vying for the mantle, while Captain America and The Falcon would focus on Falcon traveling through time to try and prevent Steve's death. Additional events in the crossover would happen in New Avengers and other related titles. Tom Breevoort liked the idea, but felt it would be redundant given that Marvel was already planning a massive Crisis Crossover for that same year.
Rob Liefeld left the Heroes Reborn Captain America series after the first arc, and had a lot of unused art as a result. He would later end up recycling said art for his Fighting American series, resulting in a lawsuit from Marvel. He had also done up a redesign for the Falcon that never actually appeared in the books, but was featured in Wizard magazine and the Heroes Reborn trading card line.
Joss Whedon had wanted to do a crossover between Captain America and Jenny Sparks, which would have involved Steve learning about both the good and the bad points of American history, and how they were reflected in Europe. Parts of the story were used in a short story Whedon later contributed to Sam Wilson, Captain America #7.
Back when The Ultimates was still in the planning stages, Mark Millar considered making Ultimate Captain America black and to have him be active during the Gulf War, but Marvel vetoed the idea on the grounds that it conflicted with the "iconic" image of the character.
American Dream from the Marvel Comics 2 line was originally going to be the daughter of Sam Wilson, before Ron Frenz decided to make her Agent 13's niece instead.
