1. Quicksilver & Scarlet Witch
Wanda and Pietro Maximoff are the twin children of the mutant master of magnetism Magneto. Like their father, they have mutant powers. Quicksilver has the ability to move and think at superhuman speeds. The Scarlet Witch has the ability to manipulate probability to the extent that she can even alter reality. The twins were first introduced as members of Magnetos brotherhood. They did not share their fathers views and when the brotherhood was dissolved, they were approached by Iron Man to join The Avengers. The twins, along with Captain America and the reformed villain Hawkeye, formed the second generation of Avengers, affectionately called Caps Kooky Quartet. The twins have been significant members of The Avengers ever since then. Both Quicksilver and Scarlet Witch are very interesting characters. Quicksilver is abrasive and impatient as from his perspective he is constantly waiting for the rest of the world to catch up to him. Scarlet Witch is insecure about not having full control of her powers. Many villains target Scarlet Witch in order to use her immense powers for their purposes.
Why should we expect them? Given that Magneto and mutant film rights are owned by Fox, it seemed unlikely that the twins would appear in the MCU but Whedon has recently announced that Quicksilver and Scarlet Witch are in the script for The Avengers 2. He had this to say about the inclusion of the twins:
You know, they had a rough beginning. Theyre interesting to me because they sort of represent the part of the world that wouldnt necessarily agree with The Avengers. So theyre not there to make things easier. Im not putting any characters in the movie that will make things easier. The inclusion of the twins in the script does not ensure that they will appear (see Wasp), but it makes it very likely that they will appear in The Avengers 2.