Según empire films:

Sequels often operate on a law of diminishing returns, but Fantastic Four has proved one of the exceptions to the rule. The first movie was, what's the term? Cack. The second was daft fun. So, for that reason, we don't anticipate the possibility of a third film with the dread that we felt about the second.

Speaking to Coming Soon, Jessica Alba said that there are plans afoot for a third part in the franchise, but it's been held up by the strike.

"The writers strike and the impending actors strike has kind of put a wrench in everything production wise," she said. "That film takes a lot of prep, a good six months of prep and about six months to shoot. With the strike, I think, maybe it's put on hold".
Tarde o temprano pasaría algo así.