User blog comment:HombreHormiga/FOOOOOOOOOOOOOOOOOOXXXX!!!!!/@comment-1671874-20130525185024/@comment-1895174-20130526031948

The weird thing is that Quicksilver's barely been announced for the Avengers sequel. Whedon's only said that Quicksilver and Scarlet Witch are in his first draft of the script. Characters included in scripts, particularly earlier drafts, that haven't been cast don't always show up. Like Gambit or Beast in the original X-Men, or Wasp in The Avengers. Then keep in mind that, even though Marvel has the right to use the characters, they don't have the rights to any of the story elements. No mention of Magneto, no mutants, there's even a possibility they can't actually use their names. Kevin Feige himself has admitted he doesn't entirely know what he still has control of.

So if a legal battle's coming, Marvel and Disney don't have much of a leg to stand on.

And I don't think it would really help DoFP's gross since it's coming out about 10 months before the Avengers sequel. There might be a teaser trailer, but we wouldn't see anything substantive by then.