The success of the Marvel Cinematic Universe has revolutionized the entire movie industry, not just the comic book genre. Following the massive success of Kevin Feige's unprecedented strategy to have a franchise connected through one overarching narrative, almost every major studio threw their hat into the ring with varying levels of success.
As the biggest game in town, the MCU has settled on a formula that works, one that's left many of the films feeling uniform and homogenized as a result. There have been countless unmade Marvel movies over the years, and some of them sound like brilliant ideas, albeit ones that Feige and his team wouldn't even dream of tackling.
The Incredible Hulk 2
If William Hurt's Thaddeus Ross hadn't made his grand return in Captain America: Civil War, then you could erase The Incredible Hulk from MCU canon completely and it wouldn't change a single thing, which is why the movie is regarded as the unwanted stepchild of the franchise.
The reason why Marvel Studios wouldn't get away with making a sequel is that legally, it's impossible. Universal still hold the distribution rights to solo Hulk movies, which is why Mark Ruffalo has been a constant supporting player. Which is a shame, because a standalone adventure picking up the plot threads in the original would be something that the fans would love to see happen one day.
Peyton Reed's Fantastic Four
The MCU has sort of tackled period pieces before with Captain America: The First Avenger, and if you count Captain Marvel's trip back to 1995, but when the rights to the Fantastic Four were still with Fox, Peyton Reed pitched the studio a trippy and psychedelic origin story that would have been true to the team's comic book origins.
Reed wanted it to have a loose narrative structure, and even named-dropped The Beatles' A Hard Day's Night, while the Fantastic Four's celebrity status would have played a major role in the story. A swinging 60s Fantastic Four with all of the costuming and musical choices that it brings would be a unique spin on characters that have yet to be done justice on the big screen, but perhaps a little too daring for the MCU.
Noah Hawley's Doctor Doom
Although the franchise has largely overcome the stigma that there's a serious villain problem, there's virtually no chance of the Marvel Cinematic Universe ever putting a bad guy front and center of a standalone movie, no matter who it is. Another one of the projects lost to Disney's takeover of Fox was Noah Hawley's Doctor Doom.
The creative mind behind Fargo and Legion has more than shown that he's capable of putting a fresh spin on familiar genres that come with their own set of archetypes, and a villain-centric comic book movie would have given the filmmaker ample opportunity to subvert and deconstruct the genre as he's prone to doing, but the MCU has always been about the heroes and the heroes only.
Deathlok
Back in the early 1990s, regular James Cameron collaborator Randall Frakes was tasked with writing a script based on lesser-known Marvel Comics character Deathlok. While the character ended up appearing in Agents of S.H.I.E.L.D. back when the show was considered canon, he was more Villain of the Week material than anything else.
These days, Marvel Studios would never even consider making a Deathlok movie, despite the undoubted potential it has. Many people have assumed the mantle over the years in the comic books, but positioning a former soldier who rebels against the government that turned him into a weaponized cyborg against his will is a little too mature to fit the MCU's established remit.
Oliver Stone's Elektra Assassin
His cache in the industry might not be anywhere as high as it once was, but Oliver Stone's greatest movies will always endure as monuments to one of the most incendiary and politically-charged filmmakers of the modern era, which is what makes it so hard to believe that he once considered a comic book adaptation.
Stone confirmed in 1992 that he was considering a movie based on Frank Miller's Elektra Assassin miniseries, but he couldn't obtain the rights despite already having his eye on up and comic actress Gabrielle Reece for the title heroine. Marvel definitely doesn't do politics, and this would have been JFK-era Oliver Stone at his most vitriolic.
Guillermo Del Toro's Doctor Strange
In the very earliest days of the MCU, the fantasy dream team of Guillermo del Toro and Neil Gaiman pitched Marvel a Doctor Strange movie in 2007, but it didn't fit the remit that the fledgling studio was looking to build a cinematic universe around.
When the Sorcerer Supreme eventually made it to the big screen, it was in a formulaic origin story that boasted some uniquely dazzling visuals. With del Toro and Gaiman at the helm, things would have been a whole lot darker, much more twisted, and quite possibly nightmare-inducing.
James Cameron's Spider-Man Scriptment
The idea of James Cameron making a Spider-Man movie in the mid-'90s remains a fascinating prospect, and while some of his ideas like the organic web-shooters made it into Sam Raimi's 2002 blockbuster, the rest of it was much better left in the pages of his infamous scriptment.
Why wouldn't the MCU want to draw inspiration from one of the industry's creative minds? Possibly because the entire treatment was one big puberty metaphor, it featured hallucinations that invoked Franz Kafka, and at one stage Peter seduces Mary Jane with vivid descriptions of spider mating rituals and then has sex with her on top of the Brooklyn Bridge. Can't imagine Tom Holland doing that.
Quentin Tarantino's Luke Cage
With the exception of Jackie Brown, all of Quentin Tarantino's movies have been original works, albeit ones full of homage to his favorites. Back in the early 90s before his surname had become an adjective, QT toyed with the idea of tackling Luke Cage.
He wanted Laurence Fishburne to play the title character, but nobody was interested in making comic book movies at the time. Based on how his career turned out, you can't imagine Marvel Studios taking the plunge on anything even remotely Tarantinoesque, from the foul language and bloody violence to the linear narratives and deep, deep musical cuts.
Joe Carnahan's Daredevil Trilogy
Fox were taking any and all pitches for Daredevil in an attempt to get another movie off the ground before the rights reverted back to Marvel Studios, but even Joe Carnahan's R-rated trilogy was too much for the studio to get on board with.
The Smokin' Aces filmmaker wanted three movies set in 1973, 1976 and 1985, that would have been rooted in the socioeconomic realities of New York at the time, but also happened to feature a costumed vigilante. The Netflix show got pretty dark, but even that would have been nothing compared to what Carnahan's period-set superhero noirs would have had in store.
Silver Surfer: The Musical
The Silver Surfer has only made one live-action appearance so far, but he was almost the subject of a bizarre movie adaptation that came genuinely close to happening, and would have been a rock opera in the vein of Flash Gordon.
Producer Lee Kramer had bodybuilder Frank Zane lined up for the title role, while Stan Lee created a character specifically for the movie that would have been played by Olivia Newton-John. If that wasn't enough, Paul McCartney was asked to write the score and songs for Silver Surfer: The Musical. The MCU could exist for another hundred years, and still wouldn't even attempt this level of sheer insanity.