3. The Blood Brothers
If Iron Man 4 was to occur after the heroes of the Marvel cinematic universe have defeated - or at the very least p***ed off - the Mad Titan Thanos, an interesting idea for the villains in Iron Man 4 would be the Blood Brothers. They are large, twin, ape-like aliens who originally appeared in the service of the Mad Titan, acting essentially as the 'bouncers' at the first base he ever made on planet Earth. When Iron Man and Ben Grimm (who wouldn't be in the Marvel cinematic universe, but could be replaced by another character) tried to interfere with Thanos' plans, he sent the brothers to kill them. Perhaps, in a movie, he could send them to carry out the same task
after Iron Man and company have interfered with his plans in a revenge plot. I don't think the Blood Brothers could carry the villainous torch on their own in a movie, but they would be an interesting sub-plot alongside another villain, and the concept of aliens is certainly not something that is out of place in the Marvel cinematic universe these days.