Well, apparently Fox and Marvel share the movie rights to Scarlet Witch and Quicksilver. Since Fox owns all mutants and the two of them are the children of Magneto, they are more closely linked to the X-Men. However, Joss Whedon considers them more closely linked to the Avengers, since both of them were members of the team since Avengers Vol 16 back in 1965. Quicksilver will be in X-Men: Days of Future Past, but Whedon was like "screw that", and he'll also be in Avengers 2, supposedly played by Aaron Johnson, who plays the title character in the Kick Ass movies. It's just that in Avengers, Scarlet Witch and Quicksilver can't be referred to as mutants (so they have to gain their powers some other way), and they can't mention that Magneto is their father.
As for the voice of Ultron, many fans want him to be played by Doug Jones, a character actor known for doing lot of acting in heavy make-up and voice-overs in lots of horror and sci-fi movies. He's best known for the Hellboy movies, as Abe Sapien, and he did the motion capture for the Silver Surfer in the second Fantastic Four movie. As for Thanos, everyone keeps saying the same thing ... James Earl Jones.
