SpikeyBits is reporting that the next four battletomes after the ones going up for preorder are rumored to be Maggotkin, Ogres, Beasts of Chaos and Gloomspire Gitz.
https://spikeybits.com/2021/09/rumors-the-next-battletomes-for-age-of-sigmar.html
Not sure what their rumors are based on.