For the Marvel Cinematic Universe specifically, casting is probably the most important aspect of production. One bad actor can destroy multiple franchises, due to the connected universe. So, rumors about castings get much more coverage than normal films, because of the impact that comes with them. Not just any actor has what it takes to be a part of Marvel. They have to have chemistry with with one of the largest group of actors ever assembled. One bad casting decision can ruin multiple franchises.
Since the first Iron Man film in 2008, there has been a constant stream of rumors for Marvel movies. Most of these stories have not been true, but that doesn't stop them from spreading again and again. People move from rumor to rumor, over analyzing every statement from actors, producers and directors. While many of these rumors are based somewhat in reality, they're likelihood is often completely overblown. Producers and directors meet with tons of actors. They're clearly not seriously considering every single one of them. It seems like the rumors that get the most attention are the craziest or worst choices, for obvious reasons. These casting rumors would've destroyed the MCU, had they been true.
15 ARNOLD SCHWARZENEGGER AS THANOS
First introduced during the final frames of The Avengers (2012), Thanos is the ultimate villain of the Marvel Cinematic Universe. Fans knew Marvel had big plans for him, and anxiously waited to find out who would play the CGI character. Obviously Marvel had to find someone with a powerful and terrifying voice.
Rumors began spreading that Arnold Schwarzenegger was one of Marvel's top picks after Jim Starlin, Thanos' creator, name dropped the actor. While he has a recognizable voice, his thick accent probably wasn't what fans were hearing in their heads all of these years. That is, unless Marvel has plans for to include Thanos' helicopter, which would give him the opportunity to scream "get to the choppa!" Luckily, Marvel went with Josh Brolin, debuted in Guardians of the Galaxy (2014).
14 MATTHEW MCCONAUGHEY AS CAPTAIN AMERICA
According to Kevin Feige, the role of Captain America was one of the hardest to cast. Eventually, the studio found Chris Evans and convinced him to take on the part, eventually becoming one of the franchise's most popular actors. According to some rumors, however, a major A-list actor almost scored the part.
Soon after Marvel first announced the first film, websites began reporting that insiders were saying that Matthew McConaughey would pick up the shield. McConaughey, known for playing cocky characters, would have struggled playing the sensitive and idealistic soldier. Marvel probably wouldn't have succeeded with a Captain America whose catchphrase is "all right, all right, all right" and likes to talk about how high school girls stay the same age. That's not a good look for the Captain... not that Evans' past is without sin, of course. Lest we forget the whipped cream bikini!
13 VIN DIESEL AS BLACK BOLT
It might be surprising now, but there was a time when the Inhumans had a bright future at Marvel. Marvel's Phase 3 film lineup originally included Inhumans. It's not clear what happened, but it ended up becoming a TV show instead. While it still meant for the big screen, Vin Diesel expressed interest in playing their leader and king, Black Bolt.
The issue is that Diesel isn't exactly a known for being a great actor, and Black Bolt is an extremely challenging role due to the fact that the character can't speak. Sure, Vin Diesel can look intense while driving a car, but he probably would've struggled to properly convey emotion using just his body language. Luckily, Diesel stuck with Groot and avoided the negative reaction to Inhumans.
12 TOM CRUISE AS IRON MAN
Back in 1998, Tom Cruise expressed interest in starring in and producing an Iron Man movie. At the time, Iron Man was a fairly obscure character. Tom Cruise never did get to don the armor or try on Tony Stark's goatee, as even A-list actors don't always get what they want. It took Marvel another decade to bring Iron Man to the big screen.
Once again, at the time, Iron Man was somewhat obscure, meaning that talent wasn't exactly storming Marvel's offices looking for auditions. Since Cruise seemingly doesn't age, his name remained attached to the project. In all honesty, he probably would've done a good job, but there's no way he would've been as successful as Robert Downey Jr, or willing to come back for all of these sequels and appearances.
11 ANDREW GARFIELD AS THE MCU SPIDER-MAN
After the disappointing box office for The Amazing Spider-Man 2 (2014), Sony seemingly cancelled plans for a shared universe of Spider-Man films and made an agreement with Marvel for the character to appear in their universe of movies. Marvel had two choices: bring the Amazing Spider-Man into its world or reboot the franchise (again).
Many assumed Andrew Garfield would remain in the role. After all, he had been expecting to make further appearances as the wall-crawler. Marvel made the right call, however, by rebooting and bringing Tom Holland in. He debuted in Captain America: Civil War (2016), putting and end to the previous version. Aside from the baggage that Garfield would've brought with him, his Peter Parker lacked the same youthful energy as Tom Holland's.
10 JOSH HARTNETT AS THOR
Bringing Thor (2011) to the big screen was one of Marvel's biggest challenges during their first phase of films. First, it had to introduce the world of Asgard, based on the Norse mythology. Second, Thor is a hard role to cast. The actor has to be arrogant without becoming unlikable, not just any weight lifter with long blonde hair would do.
Before casting Chris Hemsworth in the role, Josh Hartnett was reportedly up for the part. There's nothing wrong with Hartnett as an actor, but there's a certain absurdity to Thor that he never could've pulled off as well as Hemsworth did. He's just too serious. Also, can Hartnett even pull off an Asgardian accent? Or would Thor have sounded like he was faking a bad British accent?
9 ALEXANDER SKARSGARD AS THOR
Another name that people kept attaching to Thor was Alexander Skarsgard. If this name sounds familiar, that's because his father is Stellan Skarsgard, who starred as Dr Selvig in Thor (2011). The Skarsgard genes are pretty strong. It would've been awkward to have the two actors together in the same movie, but playing characters that are not related in any way at all.
Second of all, audiences got a chance to view Alexander in a heroic role in 2016's The Legend of Tarzan, and the results weren't great. He probably wouldn't have been a terrible Thor, but once again, it's a hard part for an actor. It's impossible to hate Chris Hemsworth, no matter how cocky he behaves. The same can't be said about Alexander Skarsgard. Not at all.
8 JOAQUIN PHOENIX AS BRUCE BANNER
It took a couple of tries to get it right, but Marvel nailed it when they cast Mark Ruffalo as Bruce Banner/the Hulk. Ruffalo was able to balance Banner's nerdiness with enough charm to deliver one of the most popular performances from The Avengers (2012). Ruffalo's Banner is incredibly likeable, but also still comes across as a nerdy guy whose emotions are a literal time bomb.
Joaquin Phoenix almost took a dose of gamma radiation, according to rumors. While Phoenix definitely could've pulled off Banner's nervous energy, there's no way that he would've been as likeable as Ruffalo. The Hulk was the least successful character going into The Avengers. Marvel needed audiences to connect with him most of all. Phoenix would have been too awkward to pull this off.
7 ALICE EVE AS MARIA HILL
The Maria Hill that appears in the movies is a slightly different character than the one from the comics. She first appeared in The Avengers (2012) as Fury's right hand man. Clearly a highly skilled agent, she also showed a tendency to question orders. Cobie Smulders plays her as much more likeable than she is in the comics, while still coming across as tough as nails.
Alice Eve was reportedly considered for the part. While she's a perfectly fine actress, she would've been wrong for Maria Hill. She might act like a executive, but Hill's a soldier at heart. It's not that Eve can't play a tough woman, it's just that Hill is the sort of character that's easy to hate. Smolders plays the character true to her roots without making her too much of a jerk, which is something that Eve would've likely struggled with.
6 JARED LETO AS DR STRANGE
Marvel wanted Benedict Cumberbatch for the title role in Doctor Strange (2016), because he was perfect for it. Initially, the actor had to turn the role down because of scheduling issues. Luckily, the studio was able to work the schedule out and bring Cumberbatch on board, not just because he nailed the role, but also because one of the other actors they were reportedly looking at was Jared Leto.
Leto, a method actor, disappears completely into a role, like the Joker in Suicide Squad (2016). The problem is that stories about him on set make him seem like a nightmare, and the performance isn't necessarily worth all the hijinks. Imagine Leto forcing Robert Downey Jr to watch him perform clumsy magic tricks on the set on Infinity War (2018) and it's pretty clear that Leto would've been more trouble than he was worth.
5 EMMA STONE AS THE WASP
The casting of the Wasp seemed to fly under the radar. That's probably because the MCU version isn't based on the original comic book version. Janet Van Dyne does exist in the films, but she disappeared long ago and the current Wasp is actually Hope Pym, the daughter of Janet with Hank Pym. Evangeline Lilly portrayed the character, who didn't even don the Wasp costume during her film debut in Ant-Man (2016).
Reportedly, Edgar Wright considered Emma Stone for the part. This would've meant that she would've jumped directly from the Amazing Spider-Man franchise to the Marvel Cinematic Universe. Since "Ant-Man" was the first time Spider-Man is referenced in the MCU, having Emma Stone in the film potentially could've come across as Marvel taking a jab at Sony.
4 NATHAN FILLION AS NOVA
When James Gunn was announced as the director of Guardians of the Galaxy (2014), everyone knew one thing right away: Nathan Fillion was going to appear in the movie. When he wasn't announced as part of the main cast, many assumed that he was going to cameo in the film. This is a rare case where a rumor ended being half true. Fillion appeared in Guardians, but in the role everyone assumed he play, Nova.
Instead, he voiced an alien that attempted to assault Star-Lord, only to get Groot's fingers up his nose. While it would've been exciting to see Richard Rider on the big screen, Fillion wasn't right for the part. Fillion is in his 40s, which is too old for the youthful Rider. It wouldn't be right to have a Nova that's older than Captain America.
3 EMILY BLUNT AS PEGGY CARTER
It seems like fate just does not want Emily Blunt to star in a Marvel movie. She dropped out of Iron Man 2 (2010) due to scheduling issues. She also almost played the Black Widow. While Scarlett Johansson's portrayal went on to become incredibly popular, Blunt probably would've done great too. Blunt's name also surfaced while Marvel was casting Captain Marvel. The role seemed like a good fit, but it went to Brie Larson instead.
One role that she was rumored to be up for that it's actually good she didn't get, was Peggy Carter in Captain America: The First Avenger (2011). Haley Atwell not only had amazing chemistry with Chris Evans, she brought a new life to the character that resulted in her getting her own TV show. That's pretty amazing considering Peggy Carter has never even had her own comic book series.
2 ELIZA DUSHKU AS BLACK WIDOW
Nobody could've predicted that the Black Widow would've become as important to the Marvel Cinematic Universe as she has. Most likely due to Scarlett Johansson's performance, the character has risen from a relatively obscure (especially to the mainstream) character to a household name. She's yet to star in her own movie, but she has played a major part in Marvel's most successful films.
While it's easy to say that nobody else could've matched Johansson's charm, that's especially true of Eliza Dushku. The former Dollhouse actress campaigned for the role, hoping a strong fan response would convince the studio. While Dushku has the action chops, she just doesn't have the same sense of humor as Johansson. Dushku's just not a strong enough actor to pull off Black Widow.
1 ALISON BRIE AS SHARON CARTER
While Sharon Carter isn't as important to the MCU as she is in the comics, Emily VanCamp has still done a good job with the role. Not only is she a good actress, she's also believable as a agent for the government's top spy organization. Also, there's something creepy about Cap hitting on the niece of his former girlfriend (or whatever he'd call Peggy Carter). It's good that VanCamp has chemistry with Chris Evans.
Alison Brie would not have worked out as well. She has a very youthful energy, which is great, but her flirting with Cap would've been way too creepy. Also, it's hard to imagine Brie handling the action scenes in any believable way. She's not a bad actress at all, but this isn't the role for her.