Both 20th Century Fox and Marvel Studios share the rights to these characters, with each studio following certain guidelines. Pietro Maximoff (Quicksilver) and Wanda Maximoff (Scarlet Witch) are the mutant children of X-Men's Magneto, with Quicksilver possessing super-human speed and Scarlet Witch having the power to control probabilities. They also appear in The Avengers comic book line, although Fox cannot mention anything that is related to the siblings' Avengers story and Marvel can't use anything in regards to their mutant lineage with Magneto.
We reported earlier this month that Aaron Taylor-Johnson is being considered for Quicksilver in The Avengers 2. Actress Saoirse Ronan has been reported to be the "model" for Scarlet Witch, although it isn't known if they will bring her on board. Evan Peters has already been locked in as the character in X-Men: Days of Future Past.
Joss Whedon also spoke about his massive success as of late, and earlier projects like his failed Wonder Woman movie that never got off the ground at Warner Bros.
No production schedule has been released for The Avengers 2 at this time.
Avengers: Age of Ultron comes to theaters May 1st, 2015 and stars Aaron Taylor-Johnson, Scarlett Johansson, Robert Downey Jr., Elizabeth Olsen, Chris Hemsworth, Chris Evans, Cobie Smulders, Jeremy Renner. The film is directed by Joss Whedon.