There is already talk about a sequel to the Fantastic Four. Michael Chiklis the actor who played the Thing as said as early as next spring or early summer shooting may begin for the Fantastic Four 2. The first one has already broke the 100 million dollar barrier domestically, and that's just for ticket sales. The toy sales are bound to make the movie very profitable. Just check out all the Fantastic Four toys.
Some cast members have also said their already signed up to number three.