If Disney were to add more "adult" elements to their Halloween events, I see no world where that would stretch beyond a "PG" representation even if the properties themselves stretched into PG-13 occasionally. In terms of what they could do similar to HKDL's mazes, I would look at their most recent Nightmare Before Christmas maze or the Nightmare Experiment mazes in terms of how far Disney would be willing to push it. Nowhere near what HKDL did pre-2016 in terms of houses. This would likely be added onto OBB as opposed to being its own separate event just with some "spooky walk throughs" that would feature a couple of show scenes and maybe a jump scare or two. I would be shocked to see Disney ever get anywhere remotely close to HHN in terms of content or format.
Edited to add: The content would likely be entirely PG. I'd be genuinely shocked to see any property that could be considered genuinely scary included. Basically, limit yourself to anything that millennials would define as "cute and spoopy," stuff that would be featured in some type of Freeform Halloween marathon, etc.