The Marvel Cinematic Universe was really born in the post-credits scene of 2008's Iron Man. In the 35-second stinger, S.H.I.E.L.D. director Nick Fury visits Tony Stark and tells him about the existence of other superheroes, as well as the Avengers Initiative.
Now, Marvel Studios President Kevin Feige has shared an alternate version of this clip, which features references to the X-Men, Hulk and Spider-Man. Considering the importance of post-credits scenes in the Marvel Cinematic Universe, nothing is ever done or said without significance. Consequently, it's quite possible that there could've been some plan to bring Xavier's mutants, Hulk and the web-head into the MCU already at work back then.
While Hulk, of course, joined the fold with The Incredible Hulk, those other two didn't make it into the MCU for a while after that.
It's no coincidence that all of those heroes in one universe might've been the dream in 2008, especially since Feige served as an executive producer for both X-Men: The Last Stand (2006) and Spider-Man 3 (2007) and held relationships with the studios. No one knew what the future held for Marvel's mutants, barring origin films such as X-Men Origins: Wolverine, but Spidey's future appeared more set in stone, with director Sam Raimi working on Spider-Man 4. In fact, plans for the fourth film were only canceled in January 2010.
So, does this mean that Tobey Maguire's version of the wall-crawler could've been eyed as a potential Avenger? While nothing has been confirmed, it isn't outside the realm of possibility. Plus, why wouldn't Marvel want an established superhero franchise and A-list actor in its ranks at that point in time?
Remember, it wasn't until The Avengers in 2012 that an MCU film out-grossed the record-breaking Spider-Man 3. If Spidey had popped up in Joss Whedon's team-up or the movies before then, the box office would've rejoiced while the audience lost their minds over a game-changing moment such as that.
Money aside, Maguire proved himself to be a popular figure among comic book movie fans. While Spider-Man 3 wasn't as praised as the first two films, no one would've begrudged seeing him return to the role in a wider universe. After all, Maguire's Spidey had defeated a host of fearsome rogues, so seeing him join other Marvel superheroes to battle galactic threats would've been the next logical step.
For Feige, the foundation laid by Raimi's trilogy obviated the need to retell an origin story while affording the opportunity to play with an experienced superhero in an established world. In other words, it was a safe bet. No one knew the juggernaut that the MCU would become back then, so Maguire's presence could've served as an entry point for the franchise while also acting as total fan service.
Of course, this alternate version of the MCU never happened. In all likelihood, Sony and Fox vetoed the mention of their characters in Iron Man's post-credits scene and other plans were made. It didn't turn out too badly for Marvel Studios, though, judging by its box office numbers and the complete cultural dominance of its films.
At the end of the day, Feige got what he wanted, albeit in a roundabout way, as Tom Holland's Spider-Man swung into the MCU (and out again), and the X-Men are set for future films. Still, it's an intriguing what-if scenario to imagine what Maguire's Spider-Man could've been in this universe if things had played out differently.