Something you could check out, and this might sound weird, are the more experimental/nontraditional Doom maps out there. If you want wide open spaces, I’ve used Firetop Mountain, ALT, Epic 2, & Phocas Island 2 for tabletop inspiration when it came to encounter design and map layout. The limitations of the engine regarding vertical movement and the lack of rooms-over-rooms mean the community has adapted in some really neat ways. I started making D&D maps exclusively in DoomBuilder because I could also make images of where the players were and what they were seeing. Lo-fi but pretty handy!
China Mieville’s Bas-Lag novels (specifically Perdido Street Station) are a good starting point for city-scale worldbuilding, if that’s what you’re wanting to do:
Bonetown: A poor district, known for its random, maze-like streets and named for the Ribs—its prominent landmark feature of a gargantuan, partially buried skeleton. The origin of the bones is unknown, but they have mystical emanations, seemingly causing increased misfortunes and accidents during construction attempts in the area. Widely considered as “thieves quarter”.
Doesn’t that sound cool?
Verticality and horizontality don’t mean much if your players get bored, though. You can describe the most beautiful stretch of land imaginable, but if there’s nothing to do, in practice it’ll feel like I-90 between Sioux City and Mankato in the winter: 4 solid hours of unending dead, grey-brown cornfields.
Pepper the map with small bits of flavor. Landmarks, small fights or NPCs, rewards for going off the beaten path… Add sub-areas (your example of a labyrinth, say your group finds a crack in the wall and they decide to bust through. Do they simply break through to the other side and continue on their way, or do they find a hidden chamber with some sick skeleton dudes to fight (maybe other adventurers trapped in the labyrinth ages ago and kept undead by the Big Bad [or maybe they sequence-broke and straight-up found the Big Bad]). As for them climbing over it, you could always add a ceiling, but wouldn’t it be more fun if they did climb over it, but remember that part in Hellraiser 2 where you actually see Hell and Leviathan? What if that? Maybe the labyrinth itself is aware of everything your group is doing, and tries to actively impede their progress, creating false exits that lead to a dungeon glamoured to look like the outside world.
But obviously, it depends on the scope of your game, and how much your players are willing to do.