While the rumors of Spider-Man appearing in The Avengers
were pure fan-boy wishful thinking, there apparently was almost a small-scale sort-of crossover
between The Avengers
and Amazing Spider-Man
At one point, the Oscorp building from the forthcoming ASM movie was going to be rendered into The Avengers
NYC skyline. I think that would have been cool. It wouldn't have been anything glaring or obvious and not distracted from the movie, but for fan-boy purposes, it still would have been enough to place the new Spidey movies firmly in the MCU despite the fact that Spidey will never actually appear in an MCU movie.