Kindle Worlds is open for submission. There are 12 worlds for which you can write fan fiction. These are: "The Foreword Saga", "Gossip Girl", "Little Pretty Liars", "Vampire Diaries", "Bloodshot", "X-O Manowar", "Archer & Armstrong", "Harbinger", "Shadowman", "Wayward Pines", "Silo Saga", and "John Rain".
Many more than were announced at first.
I personally am not inclined to write fan fiction at the moment (never say never), but I have nothing against it. In fact, I believe that it's a great idea and will enrich the literature world. It will give readers what they want and it will pay writers for their work.
Can't wait to see how it develops in time.