With those set conditions, here are a few directors capable of composing that concoction:
Wes Ball
After 2 novel adaptations with moderate budgets (slightly more expensive sequel), Ball has experience with directing a group of young adults performing crazy stunts against insurmountable odds and often clashing with each other on differing opinions. The set design is unbelievably vast for the cost of production and the creatures are viciously protrayed.
Ball also has experience with the FOX studio machine
and the transition to a $100+ million budget wouldn't be as drastic.
He can already accomplish much more with less.
Michael Dougherty
Dougherty's monster designs and effects have been some of the most memorable and original of this generation of horror flicks dominated by demonic possesions and hauntings.
His two previous films have aptly grim atmospheres and graphically violent payoffs, but executed with such comically dark reverence that adds a level fun to the serious stakes. In a Fantastic 4 film, that odd blend would provide interesting levity to both the villains and the dire situations without resorting to banter.
Doughtery holds writing credit to the upcoming X-men Apocalypse and to X-men 2,
so he's neither a stranger to this genre nor the studio.
Bong Joon-ho
Empathetic characters framing a fantastic creature or setting is Joon-ho's specialty and it is why he has two of the best monster and comic book movies on his resume. The noticeable gap in budgets and set pieces between the two films is a testament to his flexibility. Not to mention, Snowpiercer is as much a well-shot, well choreographed action scifi as it is his trademarked character-driven tragedy.
Stars Ben Grimm and the best Captain America performance to date.
Alex Garland
Finally, we get to the man who is as well-versed in science fiction as much as he is in well developed characters simply conversing with one another or performing incredible feats of valor. His directorial debut, Ex Machina, is a rather slow yet tangible drama about men and androids. This style alludes to a plausible 2001: A Space Odyssey starring the Fantastic Four. His previous film, which I learned he had far more input in directing than the drop-out director, was a lot more action packed. Dredd also had a much larger scope (even within a single location) and a larger cast of characters. The character interactions and exchanges were still immensely gratifying. The most endearing aspect of that comic book movie and one we rarely if ever get is "a day in the life" of a superhero.
...Originally intended to be published on Halloween...