Giving the guy the benefit of the doubt, but his Ultimate FF wasn't dark. It was basically Harry Potter meets Fantastic Four with the Baxter Building as Hogwarts and Reed Richards as a scientific Harry Potter. That would probably fit well with the whole "How do I get more of that sweet sweet Twilight money after the franchise is over?"
An FF with a young cast of 20-somethings would be exactly what the movie franchise needs.
Millar's Ultimate FF was far darker than the original version by Stan though. Compare Doom from that series to the Doom from the FF movies. By dark, I don't mean the characters themselves have to be grittier, but rather just the story. Keep in mind that I'm not comparing this to DKR, I'm comparing it to the Swiss Family Robinsons of comics that they were originally used as.