While the majority of the Marvel characters are owned by different Film Studios like 20th Century Fox having the rights to the X-Men characters as well as the Fantastic Four characters but according Kevin Feige during a recent interview there are some who fall into a gray area. Feige stated that Marvel Studios could use Quicksilver and Scarlet Witch in an Avengers sequel if they wanted to and could even use the Skrulls if they wanted to. He didn't say anymore than those few (presumably because they're characters not worth mentioning). He also gives a promising update on Edgar's Wright's Ant-Man in the interview. Also if you don't want to be spoiled some don't watch the interview below because they do say who the secret aliens are in the film.