So looks like the rumors from last mouth might turn out to be correct. Grey Knights are coming and the Sanctus Reach Campaign continues. Now it is possible the following images only portent a novel only, but if recent releases hold you can expect a codex and new models by the end of August.
Until then it is still possible this is nothing, but a tease.
Total speculation here, but if Grey Knights are involved, then it must mean Daemons are not far behind. Perhaps we finally get the new Greater Daemon Models? Or Grey Knights vs. Daemons box set?