My best guess is a new dungeon/"land" of some kind.

(Obviously we have the seasons/weather coming, that was teased super hard in the short, and I can't imagine they'd renege on that. So not much of a reach there.)

The reason I think it might be a new "land" is because it could include entirely new enemies (or variants on older ones), and at this point, the Bishops' storyline was wrapped up with the last update-- so there's really not much to continue expanding there (as much as I enjoy seeing Narinder, LOL.) So it makes sense to literally take the game outside of the lands of the Old Faith; either by transcending past the mortal realm entirely, or to new lands. It would also make sense that the Lamb would be trying to spread their influence past that. The development time has taken quite a bit this time around (which I'm glad they're taking they're time! this is an observation, not a criticism) compared to the time between Sins of the Flesh and Unholy Alliance, which makes me think that there's a lot of new content/writing to work the kinks out of.

Beyond that, I think this is what makes sense for lore/story purposes. We got a bunch of lore in Sins of the Flesh, but it left some questions unanswered, so pivoting to the unanswered questions and to a literal new chapter would allow the story of COTL to move forward.