The latest edition of Entertainment Weekly has confirmed two new characters which will appear in The Avengers 2. Joss Whedon has written Quicksilver and Scarlet Witch into the screenplay. Though it is very early in the process and re-writes will follow it’s interesting that at this stage they are going to be involved and a drastic shift away from them completely would seem unlikely.
Due to a licensing quirk the characters could be optioned into the X-Men films but Marvel has them available to use here as well. Quicksilver and Scarlet Witch are twin siblings (Magneto their father) and reformed villains. Both have appeared as part of the team in The Avengers comics.