I don't expect there to BE a "Mature Line" of products. Because honestly, if it were to go much beyond the type of horror that you have in CoC or that we can reasonably expect out of BoVD, WotC would be shooting itself in the foot.
Such things are pointless. You already have material saying that zombies devour flesh; if you want Resident Evil, just describe the slam attacks as tearing into the flesh with putrid nails, a character has his brain consumed after his death, etc. etc.
What I expect from the BoVD, however, is more along the lines of how certain historical evils translate into a fantasy world. What demonic plots and summoning rituals do the Inquisitors discover? How does a character's soul (whether PC or NPC) interact with these evils? My current campaign is very much concerned with these things, and to be honest I don't always have a good answer ready, nor the time to come up with one.