Games Workshop today basically announced the 4th book for the Endtimes with their secret message on What's New Today alongside a new video teaser. This book will focus on the Skaven, and we have already seen images of the new Verminlords.
Here is the secret message and a link to the video teaser.
One, two, three…
https://www.youtube.com/watch?v=rlzx9pH4kqc