Before becoming the superhero Black Goliath (later just Goliath) and getting his own series, Bill Foster started off as Hank Pym's lab assistant in a few issues of The Avengers.
Likewise, James "Rhodey" Rhodes and Carol Danvers were originally just supporting characters in the respective books they appeared in (Rhodey was Iron Man's Token Black Friend and Carol was Captain Marvel's love interest). They eventually became popular enough that they were spun off as superheroes in their own rights, with Rhodey becoming War Machine and Carol becoming Ms. Marvel (and later, the new Captain Marvel).
Carol's a twofer. After that Ms. Marvel solo title ended, she definitely fell away from A-list status for a long time. If you grew up with comics anywhere from the 80s to 2005-ish, you probably think of Carol Danvers as "who? Oh, the one Rogue stole her powers from" and find it hard to imagine her as a main character. However, real effort has gone into pushing her from that to beyond her status in her original heyday; she's now Marvel's Wonder Woman.
The Black Widow was just a supporting character for many years, not a full Avenger (with good reason — she started out as a Russian spy, and this was the Cold War). She's now been an Avenger long enough that it's hard to think of her otherwise, and adaptations reflect this. The Avengers bumped her all the way up to founding member status!
Similarly with Hawkeye, who started out as a reluctant Iron Man villain, only to become the Avenger with the second most appearances on the team (behind only Captain America), founding leader of their West Coast branch, headliner of the Solo Avengers spotlight anthology, and present in every animated, and now live-action, adaptation of the team.
Spider-Man: Eugene "Flash" Thompson was once a Jerk Jock who bullied Peter Parker, but was a major fan of his web-slinging alter-ego. After the shift away from high school, Flash tended to stick to the background before becoming a soldier. After an accident cost him his legs, he got a second chance in serving his country. He became the Venom for a time, has been a member of the Secret Avengers, and even dated Valkyrie. Now, that's impressive.
Squirrel Girl was originally a one-shot character created for a gag story where she teamed up with Iron Man to fight Doctor Doom. She went close to a decade without appearing again until she got revived in 2005, and has since gained enough popularlarity to become an Avenger and get her own ongoing series.
Gwenpool began life as part of a variant cover gimmick that featured Gwen Stacy dressed as different Marvel heroes (Captain America, Iron Man, Thor, etc.) The cover showing Gwen dressed as Deadpool was an instant hit, and the design proved so popular with cosplayers that Marvel brought Gwenpool into their official canon as part of the All-New, All-Different Marvel universe — first appearing in a Howard the Duck backup story, she then proceeded to headline a holiday special, before finally getting her own ongoing series.
Before he was revealed to be the true identity of the Green Goblin, Norman Osborn originally appeared in the background for several issues as a member of J. Jonah Jameson's businessmen's club, and occasionally seen talking to Jonah. He finally received a supporting role in one issue where it's revealed he cheated his ex-business partner, the villain of the issue, had another small but important role putting a hit out on Spider-Man, and then came the issue where the Green Goblin was finally unmasked. The rest is history.
AIM Scientist Supreme Monica Rappaccini wasn't quite an extra, but she was originally created as an antagonist for her daughter, Carmilla Black/Scorpion, with no sign that she was expected to take on any role in the wider Marvel universe. However, as one of the few human members of evil organization AIM to have any characterization, she started cropping up in other comics, Marvel's Avengers and eventually the MODOK TV show — whenever they needed a face for the organization, or just an evil biochemist, or even sometimes when they just needed a slightly morally ambiguous one, long after Carmilla had become a footnote in Marvel history.
Rio Morales, Miles' mother, wasn't a very prominent character in Into the Spider-Verse, only appearing in a few scenes. However she gets far more screentime in Across the Spider-Verse and a good chunk of the movie is spent on her relationship with Miles, with it serving as one of the film's emotional cores.
The Spot is a very literal example. In the first movie, he is a background character who gets maybe three seconds of screentime as the receiving end of a blink-and-you'll-miss-it minor gag where Miles throws a bagel at him. In the sequel, he serves as a seemingly one-off antagonist character who Miles fights at the start. By the end, however, he evolves into a multiversal-level abomination dead-set on ruining Miles' life.
In Spider-Man 3, Elizabeth Banks and Ted Raimi receive billing after their smaller roles in the previous films led to slightly bigger ones in the third.
