This is a great visual guide by The Geek Twins, showing which studios own which Marvel characters, as it can get quite confusing! It clearly shows why Spider-Man, the X-Men, or the Fantastic Four won’t be part of any future Avengers movies, as the rights to those characters are owned by different studios. It’s a shame, because it would be so awesome to see more cross-overs. I would especially have liked to see Spider-Man as part of the Avengers.
Stan Lee is involved within everything though, which is cool. Onwards!