How much change can go into the new Fantastic Four movie before it stops being an adaptation - and does that even matter?
You'd think that in this age of so much media being adaptations of properties from various other media, "popularity" would be an easy thing to ascertain: Look up whatever makes the most money or has the highest consumer-awareness rating and make a movie out of it. But in Hollywood, where fortunes have been made off of dime-store books no one had ever heard of and lost on "sure things" starring the biggest actors on the planet, remains wary of playing strictly by the numbers. Instead, a unique alchemy of numbers, "analyst" input and (most powerful of all) the personal/professional biases of executives gets to tell the tale.
That's why, when Tim Burton's Batman hit huge in 1989, studios rushed to capitalize on its success not by quickly setting up movies based on Spider-Man, Wonder-Woman or even Superman but instead by hurriedly ordering big-screen updates of... 1940s comic-strip and radio-serial heroes like The Shadow, The Phantom and Dick Tracy - "comic heroes" older executives remembered had worked on film before (Batman, too, had been a 40s serial hero - a viewing of which at a Playboy Mansion party is said to have inspired the making of the Adam West TV series.)
This goes part of the way to explaining why The Fantastic Four is still considered a hotly-contested media property. Granted, in a world where Rocket Raccoon and Drax the Destroyer are the leads in a box-office topping smash, anything from the Marvel Comics canon is now considered a hot item. But FF was bought by Fox (and subsequently made into two initially-popular but now largely forgotten features) in the days when Marvel was desperate for any deal it could make, over properties like Captain America, Thor and Iron Man now considered billion-dollar icons.
But even then (i.e. the early-2000s), Fantastic Four wouldn't have looked like a comic you'd scramble for the rights to: The books haven't sold particularly well in decades (even during periods where it's writing/artwork were critically-acclaimed) and the closest thing it has to a "hook" - its heroes are named/powered more like the fixtures of the 1950s scifi/monster pulp yarn than conventional costumed adventurers - hasn't been novel since it debuted in the 60s. Their overall presence in the rest of the Marvel Universe, even, often seems less to do with their actual stature than to elements of their mythos (the "unstable molecules" handwavium, villains like Galactus and Doctor Doom) that quickly bled into everyone else's books. So what's the appeal (to studio decision-makers, fans or both?)
They're recognizable, basically. Like Wonder Woman, they've spent decades being "name" characters whose historic importance (they were Stan Lee and Jack Kirby's first superhero collaboration, generally-credited as both the "rebirth" and modernization of the genre) and pop-visibility (they've had a slew of cartoons and almost-innevitably pop up in the 'toons of others) far outpaces their on-the-ground saleability. To the degree that most audiences know the name "Fantastic Four," they know it means a Marvel superhero team - and now, more than ever, that should mean something you can sell without having anything else familiar to offer.