Warning - The rest of this article contains spoilers for Spider-Man: Across The Spider-Verse.
The Spider-Man situation between Sony and Marvel Studios is well documented amongst even the most general of audiences. Sony has the rights to the character and has no plans of letting go.
Other than a brief stint between contract negotiations, it has not impacted the webhead's presence in the MCU. But as the Multiverse continues to be a dominating force in Hollywood, the lines between this universe and that universe are more blurred now than ever before.
1.) Spider-Man: No Way Home Shoutout
In Spider-Man: Across The Spider-Verse, Spider-Man 2099, played by Oscar Isaac, is in charge of protecting the Spider-Verse from collapsing by making sure certain events play out the way they are supposed to (sound familiar?). While explaining the Multiverse, Miguel mentions "Doctor Strange and that punk from Earth-199999" did something to interfere with that goal.
This is a shoutout to the events of Spider-Man: No Way Home when Strange and Spidey unintentionally disrupted the Multiversal balance by bringing different universes' Spidey villains (and Spider-Men) into the MCU. This also set No Way Home as the precedent for any Multiversal mishap with villains getting thrown across different universes anytime there is a knot in the thread.
2.) The Sacred Timeline
For MCU fans, this is without a doubt the biggest Easter egg in the movie. When Miguel explains the nature of the Spider-Verse, he first shows an example of the vast Multiverse and it is none other than the fractured Sacred Timeline from Loki.
This confirms too much for the future of the Marvel Cinematic Universe and the Sony Spider-Man Universe. This Easter egg proves that not only is Sony allowing Marvel Studios to use its prized possession, but it is also willing to play ball.
As Avengers: Secret Wars approaches, the more universes tied to what is happening in the MCU, the bigger that movie is going to be.
3.) The Return of Donald Glover
While the Sacred Timeline is an MCU fan's biggest Easter egg, Donald Glover's return as The Prowler is the biggest cameo. After a brief appearance as Aaron Davis in Spider-Man: Homecoming, Glover is seen in Across The Spider-Verse as a fully developed and captured Prowler.
So many things make this cameo great, and the reasons stretch beyond this movie. But with a new Spider-Man, a Mile Morales movie, and Spider-Verse 2 being a box office darling, look for Donald Glover to return once more as Aaron Davis to support whoever ends up being the live-action Miles Morales Spider-Man.
A role Glover surely would have been the favorite for in the 2010s.
4.) J.K. Jonah Jameson
In theaters, there have been four leading Spider-Men, four Aunt Mays, three Uncle Bens, and a plethora of Stan Lee cameos. But only one man can be trusted with one of the most iconic Spider-Man characters. That man is JK Simmons, and that character is J. Jonah Jameson.
In Across The Spider-Verse, Simmons can be heard voicing every instance of JJ Jameson in every universe. The same way he portrayed the MCUs JJJ, no questions need to be asked. He has perfected this craft for over 20 years now.
5.) Andrew/Tobey Archival Footage
While these are not MCU Easter Eggs, the most recent appearance from Tobey Maguire and Andrew Garfield as their respective Peter Parkers in No Way Home really ties Sony and Marvel Studios together with Across The Spider-Verse.
Footage from Spider-Man and The Amazing Spider-Man was shown to highlight the canon events in a Spider-Man's life. This helped fans feel that this animated spectacle is just as important and tied into the great Spider-Man movie universe.
Everything Is Connected...Like A Web
These Easter eggs, cameos, and callbacks may seem like fun fanfare on the surface. And in a lot of ways they are. But these sorts of connective tissue strengthen the audience's perception of what is connected and what isn't.
There is going to be a moment just like December 2021 when audiences see their favorite characters from all different studios and franchises coming together on screen. These types of world-building moments are training their brains to be ready for it.
No matter how futile of a task that may be.