Love the idea of one massive, intriguing dungeon like the Durlag's Tower from BG1. From all IE games loved that one the most.
But what I really want to see in PE is a sinister and twisted location like famous Castle Drachenfels from pen-and-paper Warhammer 1ed. module.
What was the most exciting of it?
1. One nasty, enourmously powerful and diabological adversary.
The one that PE players should remember a long time after they finish their games. This final enemy could have i.e some political (mostly by intimidating way) connections, be really really old and have vast knowledge of various things, the nature of souls for sure. The option to pact with this entity instead of killing it would be a thrilling twist in main character adventures. This could be beneficiary for players if devs appreciate developing tiers for evil characters.
2. A twisted, insanely dangerous and tricky dugeon/castle with deadly traps and horrors.
I'll appreciate presence of traps in PE a lot, especially unusual and not obvious ones. I really love mind-controlling and maddening ones, more narrative in nature than instant. The ones that lure the character into something dark and evil. Oh I want this, sooo much.
3. A well placed and motivated enemies, not just mindless hordes that main enemy constantly throwing at You by waves. Maybe some tormented souls trapped forever in this haunted dungeon, seeking a way out or just so insane, they only wish to rip the poor mortals apart. Maybe not necessarily hostile inhabitants with minds twisted from sheer presence of this great evil, but willing to share some important lore with main character when carefully approached.
So, yeah, it should be a great story around this dungeon, not just enemies, corridors and dozens of similar traps. It should be twisted, intriguing and by most unexpected. It should have many dramatic moments and memorable NPC's.
If devs could create such a location, I'm on for it!