I think that there are certainly contenders who can pick up the gauntlet dropped by the Lord of the Rings and Harry Potter franchises. Unfortunately, it's that mysterious combination of the popularity of the book and the quality of the movie that will color the landscape of the future of fantasy cinema. Logical successors like the Lemony Snicket books and the "His Dark Materials" trilogy stalled out for various and sundry reasons. Eragon and The Last Airbender were pretty soundly panned by critics and audiences alike, making franchises pretty unlikely. Narnia is chugging along, but (as you say) the series is pretty far along and the majority of the movies will focus on characters other than the Penvensies from the first three films, thus making it a challenge to keep the interest of the moviegoer through four more movies (particularly given the gradually diminishing returns of the first three).
The logical successor from the "Harry Potter" end of things would be the Percy Jackson books. A sequel has been greenlighted and that sequel won't have the Potter films to compete with this time around.
As for more adult-oriented sword-and-sorcery, the success or failure of the Hobbit films will likely determine how willing studios will be to pony up the cash for similar efforts. Some good areas to explore would be the Wheel of Time series by Robert Jordan, the Pern novels, and Terry Brooks' Shannara and Kingdom of Landover novels.