Dark Life won't hit bookshelves until May, so hasn't hit a Harry Potter level of mania or a "Will he adapt it properly?" fear. We don't even know if the book is any good. After all, plenty of novels are snapped up pre-emptively, and some of them never make a ripple on the shelves or the screen. I suspect Dark Life has been snapped up because it clearly taps into that new, very much sought after market of "tweens." Every studio wants a Twilight as much as they want a Harry Potter or a DC or Marvel, and Disney lacks all of the above.
Falls' book is set in a not-too-distant future where rising oceans and natural disasters have caused some people to homestead on the ocean floor. The environmental changes have also led some kids to develop weird powers. The story centers around an underwater boy and a teenage girl who meet, become friends, and team up to stop a government conspiracy. I imagine they might also fall in love along the way, since this is the first of a series. It's unknown whether Zemeckis will use his beloved motion capture style; but considering the level of fantasy that's presented in just the synopsis, I can't believe he'd want to do anything but motion capture. It could be his Avatar -- and I'm sure that is being name dropped as readily as Twilight around the dark and hallowed offices where such things are signed.