Fox sees the increased revenue from Iron Man 3, Thor 2 and likely Cap 2 after The Avengers and wants a boost in what will be a competitive ten year period starting in 2016. WB, Disney and Sony will be pumping out teams ups and spinoffs with their characters and Fox won't be left behind in the arms race.
Perhaps the mystery film will be more than a FF/XM team-up and would feature other characters for their own spinoffs to come in 2019-2022. To them a FF/XM team up may boost a third FF film, a solo Silver Surfer or help an X-Force film earn $50 million more.
THOSE GUYS ARE INSIDE YOU BUILDING A PIECE OF ****, ETHAN!
They're inside you building a monument to compromise!
Rick and Morty