Any one who is a fan of the Marvel Cinematic Universe must know about the 2008’s Iron Man. This is the very first movie in MCU Phase 1. Recently, a deleted scene from this movie has been revealed. This scene has never been seen before and it appears to have Spider-Man and X-Men references.

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Marvel wanted to have Spider-Man in the MCU way back from the beginning of it

During the Saturn Awards last weekend, Kevin Feige revealed a post-credits scene that no one has even seen before. The scene is from the original Iron Man movie. Feige shared that he had never had any intention to show that scene to anyone who were outside of Marvel.

Feige announced at the Saturn Awards "I'm about to show you a deleted scene that has never been seen by anyone before right now”. In the scene, you can see Nick Fury (portrayed by Samuel L. Jackson) talking to Tony Stark. They discuss about the Avenger Initiative alongside how Tony is handling "gamma accidents, radioactive bug bites, and assorted mutants." Those are obviously references to X-Men and Spider-Man.

We can see the implication here, which is Marvel was fond of having X-Men and Spider-Man as parts of the Marvel Cinematic Universe. And that is not just recently but all the way back from the beginning of the whole MCU.
That didn't work out well completely, however. The Spider-Man portrayed by Tom Holland took part in some of the MCU movies. Still, Holland is now out of the MCU as Disney, who owns Marvel, and Sony, who owns the movie rights for Spider-Man couldn't come to agreements.

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Though X-Men belongs to Marvel now but they still haven't announced any plans with this franchise yet

Regarding X-Men, Disney owns this whole franchise as it bought most of the entertainment assets of Fox. Still, there are currently no plans for the X-Men to become a part of the MCU. On the bright side, Ryan Reynolds (Deadpool) has hinted that there may be a chance Deadpool would join the MCU.

The real post-credits scene that we saw in Iron Man ends was when Fury says, "I'm here to talk to you about the Avenger Initiative."