It has been confirmed by Joss Whedon that villain siblings turned heroes Scarlet Witch and Quicksilver will be new additions to Avengers 2. Whedon added they would represent an aspect of the world not entirely thrilled with the Avengers. Nothing says this statement means they'll be straight up villains (Thunderbolts/H.A.M.M.E.R.) but sounds like they'll have their clashes with the team. I assume the brass at S.H.I.E.L.D. and World Governments could be unhappy that they have superheroes running around without leashes. Which might be the seeds of Civil War which has been teased before.
Apparently, Marvel might be using Saoirse Ronan as a possible model for Scarlet Witch. Not sure if that means they'll be looking for an actress her age or someone with a similar mix of action/dramatic roles. Marvel will likely hold off announcing any casting for Avengers 2 and Phase Three's Ant-Man until Comic-Con 2013 in July. Yet, I have a few names that could make the mutant siblings really standout.
SCARLET WITCH (WANDA MAXIMOFF)
RUTH WILSON (Luther, Lone Ranger, The Prisoner, Anna Karenina): Ruth Wilson has some serious Marvel connections playing roles alongside actors Idris Elba, Tom Hiddleston and Hayley Atwell. Even actor Ian McKellen who plays her character's would be father Magneto. She had been linked for possible talks for a role in Avengers 2 months back. Disney might be looking to up her profile as Ruth already is the female lead in their Lone Ranger tentpole this Summer.
JESSICA CHASTAIN (Zero Dark Thirty, The Help, Lawless, Mama): Jessica almost played Maya Hensen in Iron Man 3 but ultlmtely took the lead in Zero Dark Thirty instead. While earning an Oscar nomination for ZDT, she did state being open to working with Marvel in the future. She has the talent and popularity to bring enough life to the character.
JESSICA BROWN FINDLAY: Jessica was in the running for a role in Captain America: The Winter Soldier and also had a small cameo in Iron Man 3 with her scenes from Downton Abbey playing in the background. Marvel seems to love television actors for new roles and Witch would be a role that would likely become a staple for future Avengers sequels. Similar to hiring Tom Hiddleston as Loki, Findlay could balance out playing baddie and anti-heroine.
QUICKSILVER (PIETRO MAXIMOFF)
BENEDICT CUMBERBATCH (Hobbit Trilogy, Star Trek Into Darkness, Fifth Estate, Sherlock): Benedict has long been a fan favorite to play Doctor Strange but his current run of playing villains could make him an excellent Quicksilver. To be honest I could careless about Quicksilver but someone like Ben could really give substantial dimension to him. His turn in Star Trek Into Darkness proved his ruthlessness as a villain which might be needed for the Avengers sequel to be darker than the previous one. Marvel has a history of casting Star Trek alumni with Chris Hemsworth (Thor) and Zoe Saldana (Gamora) taking roles. Other cast members such as Chris Pine and Alice Eve had been in the mix for the leads in Captain America: The First Avenger. Even Simon Pegg has stated interest in playing a villain in Edgar Wright's Ant-Man.
RYAN GOSLING (Drive, Place Beyond The Pines, Only God Forgives): Getting Ryan Gosling is a bit of a longshot but I think he could work well with Joss Whedon and the cast. He's a very dedicated actor with dramatic chops but might be harder to lure into a Marvel contract. The fact he's almost entirely sworn off comic book films would be a giant get for the studio. Ryan's got a solid background with stunt training (martial arts, weapons, stunt driving/motorcycling) thanks to his numerous action films such as Drive, Only God Forgives, Gangster Squad and The Place Beyond The Pines.
CHARLIE HUNNAM (Sons Of Anarchy, Pacific Rim, Green Street Hooligans): Charlie is very suitable actor that could work within the Marvel blueprint. While he has Pacific Rim on the horizon this Summer it hasn't made him a household name at this point. Similar to Cumberbatch, Charlie has a massive following thanks to his series S.O.A. Originally, Charlie had been one of the few candidates to play Thor before Chris Hemsworth ended up being cast in the role. He was recently linked for possibly playing The Flash in Justice League but that film is being put on hold until the box office returns for Man of Steel come in. Marvel would be smart to sign Hunnam up before Pacific Rim 2 or Justice League gets a greenlight.