I've not seen it (and don't plan to) but doesn't the movie end with them basically becoming the Fantastic Four?
So I suppose really could they not still do a sequel with same cast, but have it be a very loose sequel? They could totally change the tone of it, the costumes, the look, style, everything really, but still have it be a "sort of sequel"
So, sort of like The Incredible Hulk, but with the same cast?
There's precedent for such things. There have been movies that were very loose sequels to the originals; Highlander II is an example of one that was worse, but there are probably instances where a poor first film was followed up by a superior second installment that largely ignored the original. Heck, the Mission: Impossible film series wasn't any good until the third film, and both the second and third films felt like complete reboots despite keeping Tom Cruise and Ving Rhames.
It's been done on TV as well. The pilot of The Six Million Dollar Man had a number of elements that were disregarded or retconned in the later series -- Darren McGavin's character was replaced with Oscar Goldman as the man behind Steve Austin's bionics, Steve was changed from a civilian astronaut to an Air Force colonel, Steve's love interest from the pilot was retconned out of existence and replaced with a one-shot guest star in a later episode, etc. And the M.A.N.T.I.S. series kept the lead character, actor, and premise of the pilot movie but completely rebooted the continuity and replaced every other character and story element (though the series was immensely worse than the pilot).