It seems we have another good hint that we will be seeing pre-orders next week for the Genestealer Cult Codex. Tooth and Claw, a World of Vigilus release is sold out in both the UK and the US+ Can, and if still available elsewhere will be gone when supplies run out.
https://www.games-workshop.com/en-US/Warhammer-40000-Tooth-Claw-2018-eng