Re: The Importance Of Shield
The problem with cameos is that the character, if being played by a major actor, needs to have a solo film or a larger role up-and-coming. Otherwise you have to cast a nobody and then many years down the road recast. These little references can be tricky things to pull of logistically.
I think the Avengers movie would have been too early to introduce Pym. It worked introducing Hawkeye in Thor because the actor was guaranteed a larger role in The Avengers the very next year.
The point is that yes, Marvel does have a lot of opportunities, but they have to be careful with how it is executed. As a counterpoint, look at all the references to characters in the Fox X-Men movies that later end up being retconned due to poor planning / foresight.
Restraint is better than future continuity issues.