The lightning-fast son of Magneto is in high demand in Hollywood nowadays! Not only has Joss Whedon confirmed Marvel's speedster will appear in the Avengers sequel, director Bryan Singer just announced that actor Evan Peters has been cast as Quicksilver in X-Men: Days of Future Past!
Wow. Well, this is interesting. There's absolutely no word if Peters is even being considered to reprise the role in Avengers 2, but I'd guess it's pretty unlikely. Sure, this could be the first secret step to Marvel and Fox actually playing nice and merges their two cinematic universes, but I think it's much more likely that both Quicksilvers will be portrayed by two different actors that never once reference the others' situation (as legally both Marvel and Fox can use Quicksilver, but the former only as an Avenger with no mention of mutants or Magneto, and the latter must have absolutely nothing to do with the Avengers). Also, who knows whether Peters is playing Quicksilver in the '70s or a modern Quicksilver? It might not even be feasible,.
Still, it would ne a nice concession to the fans if the studios managed to work out a way where the two movies didn't completely contradict each other, but that might be too much to hope for.