places above, below, between

I dream of a zone which is a vast forest, where you could not only travel about on the ground, but also climb into the arbosphere and follow an intricate maze of skyway paths. The tree level wouldn't simply be another flat level overlaid on the ground level, but a 3 dimensional space, one where paths criss-cross above and below each other, where shortcuts are available by carefully jumping down in just the right spot, where apparent dead ends reveal ladders that open up whole new opportunities. Mix into that the occasional high protuberance from the ground level such as a hill or building to further complicate the potentials.

Floating above the tree level there could also be a cloud level, where gas leviathans lazily drift from place to place, and you'd need to now take into account the fourth dimension: time. Once all that is happening, build in an underground level too. Tunnels, caverns, precarious paths along cliffs. Some tunnels would wind upwards, exiting on cliffs overlooking the tree level.

The underdeep evokes memories of Wailing Caverns, with it's interesting possibilities of different paths, of having to leap across a gap to get to the next area ... which is most unlike Karazhan - while that place has used the vertical dimension you can't do a whole lot with it. There are no short cuts I know of available, and unless you have a parachute cloak or some magic skill a wrong step would usually be fatal.

Different sub-zones or gateways would be controlled by different factions, necessitating careful balancing of reputations. Certain paths should be mutually exclusive, determined by which rivalrous faction you have reputation with.

An explorer's paradise.