references:
* the Hulk (dont need to see him, but a nod or reference or even like a tv news report on "Bruce Banner = Missing" or something)
* the Avengers (im sure since SHIELD and Nick Fury will be a part of it, the idea of the avengers will be brought up just like in IM1 and TIH)
* S.H.I.E.L.D. (duh, they r gonna be in it)
* maybe, like, on tony's holographic computer thing you briefly see a holographic outline for Ultron or something
* Red Skull, Capt America or something, not seen but heard. like fury mentions ww2 and how long superheroes have been around or something like that
* i doubt it cuz fox owns F4... but make Tony Stark mention Reed Richards