For theme I'd probably go with something old school, like a cave or dungeon/castle. Difficulty would be average to high (or at least higher than the current). Not stupidly punishing or hard but something with actual threat and teeth like Aurum Vale or even Stone Vigil with patrolling mobs and mobs you can't just easily see coming (a la the Ice Sprites). You should be rewarded for knowledge of the dungeon and be encouraged to not make any silly mistakes.

As for mechanics, I'd like to incorporate some sort of light puzzling or switch usage a la Qarn or even Hala Tali. Nothing taxing or even necessarily on the main path but just something to give the place a bit more life than the usual hall/boss/hall/boss.

I'd go with a fairly straight forward main line path but optional areas with reasons to explore a little that actually mattered to some degree (extra unique gear rewards, minions, maybe even a small chance at a mount, etc.).

I'm toying with the idea of the slowing puddles of Toto Rak as I actually like what they were trying to do there but without doing it in the most annoying way possible. Some sort of environmental danger would be a requirement, though.