The best easter egg, or hint about the Avengers is the following one:
We already knew thor and captain america would be in the avengers, so its not surprising that we saw them in Iron Man 2. This is why the following Hint is so much beter, we didnt expect it.
CLICK ON THE IMAGE BELOW TO SEE IT.
That image appears on a TV right before the racetrack race-car scene in Iron Man 2.
Clicked on it and saw nothing? Well the 2nd name on that list of people competing in the list belongs to a BRITTISH car named CHAPMAN.
Joey Chapman is the third generation UNION JACK. Yes, UNION JACK!!!
Union Jack is the Captain America Brittish spy-villain.
Obviously Union Jack will be a Captain America villain, and his 3rd generation version (maybe grandson) will be in the present, where the Avengers take place.