1. Captain America wasn't even part of the original Avengers
The first Avengers team was actually comprised of Ant-Man, The Wasp, Iron Man, Thor, and The Hulk. It was only in the 4th issue of the comics when the Cap joined the team and was granted founding member status as a replacement for The Hulk.
2. Spider-Man refused membership to the Avengers
Despite willingly (and stubbornly) wanting to immediately fight with the grown ups in the MCU franchise, Spider-Man actually declined Tony Stark's invitation to be part of the team in Spider-Man: Homecoming. This mirrors his refusal to join the Avengers in the comics as well. But aside from Spider-Man refusing to join the club, he was also denied entry multiple times, once by Tony Stark and another time by the government.
3. Scarlet Witch and Hawkeye were both villains
Scarlet Witch and her brother, Quicksilver, were both introduced in the comics as villains. Both of them worked for Magneto in the Brotherhood of Evil Mutants, a group that included Sabretooth and Juggernaut among others. Hawkeye, on the other hand, was originally an enemy of Iron Man but mostly because of being misguided in those early years. These three were brought in as replacement Avengers when Iron Man, Thor, Ant Man, and The Wasp all left the group.
4. Black Widow is hella old
Black Widow looks absolutely stunning in both the movies and the comic books. But did you know that, at least in the comics, she is really old? Her backstory tells of our favorite leather-clad spy being alive as early as World War II. That puts her at about the same age as Captain America at at least 70-plus years old!
5. Nebula was the Endgame in the comics
Nebula, daughter of Thanos and sister to Gamora in the MCU played a pretty substantial role in the last two ensemble movies. But in the comics, Nebula was actually the one to remove the gauntlet from Thanos's hand and was the one to bring back those who vanished after the snap. Thanos even teamed up with a bunch of heroes to remove the gauntlet from Nebula.