If I were RM, I would do something involving a murderous and corrupt cult. Could be modern day or 60s-70s.
EDIT: Or! In the vein of the slasher theme, maybe the characters are being hunted by a group of serial killers. Or a serial killer showdown. I watched a short film the other day which had the premise of "What if
Ted Bundy picked up a hitchhiking
Aileen Wuornos?" It has potential, but I guess that is more action-oriented than horror.