I don't know; I'm honestly really not sure. "Any Easter egg"? That basically covers the entire gamut, from a short, throwaway line to something potentially much, much larger like a shot of the X-mansion or Baxter Building on a news report or something or even casting a character, which, in addition to the casting process, would potentially include costume design. I might have gotten off-track with the last one; are cameos considered Easter eggs?
And then you probably need to consider other logistics with shooting. Are the people involved in shooting it busy? Would the assistant director or somebody suffice, or would it have to be the Russos? Do they need special permission to shoot in a location or is a certain location (i.e., snowy Canadian wilderness featuring a certain berserker) not available at the time? Do they have to storyboard anything or any other preliminary stuff? And there's probably so many other things I'm forgetting, too.
To be honest, it could honestly be pretty simple. An improvised line not even in the script like RDJ's "I am Iron-Man." In fact, I seem to remember much of the dialog in the movie was actually improvised. And then consider that these scripts are never necessarily "completed"- big opportunity there.
Some of the stuff they could do- just a still frame of claw marks on a tree or a silver meteor shooting through the sky- I feel is so simple I could do it myself.
I don't have a background in film by any means, so... maybe someone in film or something film-adjacent could contribute. Do we have anyone like that here on the forum?