No, he hasn't taken over At The Mountains Of Madness from Guillermo Del Toro: The Mountain** is a different kind of horror project and Fox has just picked it up for Ben Stiller to direct.
It's a spec script by Helen Childress, a blast from Stiller's past, since she wrote Reality Bites, which he directed in 1994. The Mountain is described by Deadline as "a sophisticated horror premise in the vein of Rosemary's Baby". Very strangely, it's apparently based to some extent on Edith Wharton's 1917 novel Summer, which in no way, shape or form would seem a likely basis for a scary movie.
Wharton's book is about Charity, making a rather more respectable living than her prostitute mother as a small-town librarian in New England. But her world is turned upside down when she starts an affair with architect Lucius Harney, who's engaged to a society girl. Charity gets pregnant, and flirts with the idea of returning to the poverty-stricken mountain community of her roots to follow in her mother's footsteps, until her protector Lawyer Royall steps in and marries her. The book shares many of Wharton's themes (women's place in early American society; sexual awakening and peer disapproval) with later works like The Age Of Innocence and The House Of Mirth, and has several plot points in common with Ethan Frome.
It definitely isn't the story of "a young woman who struggles to confront her destiny after stumbling upon a mysterious object that forces her to examine the secrets of her past", although there is a brooch that serves a significant purpose. So quite what's going on here is unclear. Some sources are suggesting that Childress' script is somehow a sequel to Summer, while others assume it's just a very loose reworking of the book, or an entirely new story that for some reason uses Wharton's characters.
When we know more, we'll tell you. The Mountain is some way off yet, since it's still at the development stage. In the meantime, Stiller and Childress are also collaborating on an adaptation of Ezra Jack Keats' 1960s kids' picture book The Snowy Day, and Stiller's also juggling The Secret Life Of Walter Mitty, a David O Selznick biopic, Neighbourhood Watch and, believe it or not, Rentaghost.
Have a read of Summer at Project Gutenberg, and tell us how frightening you think it is.