I think a gloves-off-hardcore-Mythos campaign would lead to a new model for AP's: One where each adventure starts at first level because it's assumed the party for each previous installment was consumed completing it.
Assuming that is not the plan, (and based on the comments reported from the panel) I'd say we're going to see something a little more in between the options of "CoC style" and "Pathfinder traditional".
I would guess that the Horror Book (I'd like to see it named "Horrific Adventures" but that's just me) adding in things like insanity/corruption rules will give DM's a chance to do interesting things to a party over the course of 15 levels beyond just outright killing the PC's or trying to make them run away. I suspect it will be about as key to running the AP as Mythic is to Wrath of the Righteous, Technology is to Iron Gods, and Ultimate Campaign would be to someone starting a new Kingmaker game. Maybe not technically essential, but a good idea nonetheless.