I'd definitely add some new enemies, especially those terrifying concepts of the Guardians from Creating a Champion.
(these guys)
As for what already exists though...
I kind of think the overworld will be heavily changed by perhaps things rising from underground.
This could let Aonuma put huge holes in the world where dangerous and scary enemies (like poes and redheads, bubbles and gibdos etc) could pour out of and fill the overworld.
Some of the things rising from below could be long forgotten dungeons (perhaps created by the Zonai) and they could be full of creepy things like how the Shadow Temple in Ocarina was a Torture chamber or the Arbiter's Grounds in TP was a place of execution.
I'm of the opinion that were was nothing really creepy or scary in BotW at all, but the trailer for BotW2 proves the style can be both, it's just up to Aonuma to have some really good ideas for it.