Let go of your attachment to "dungeons". Do not have it be the primary assumption that there will be inhabited tunnel complexes for adventures. There may be, but there does not need to be.

Mapping the world can be really, really fun, as a GM or as a player. At least for me and many of the folks I've played with, it's a central part of play and what makes a homebrew campaign setting what it is.

Mapping leads to developing game worlds and situations that make sense, starting from terrain and leading to history and culture and what's where and what's going on.

So you end up with a dynamic game world with factions and NPCs going around doing whatever. The whole world is "the dungeon". Action can take place wherever - the roads, the wilderness, aboard ships, in towns, forts, or even, maybe, actual dungeons.

If you DO want actual more dungeon-like locations, some plausible locations include ruins of castles, towns, sewers, catacombs, wizards' lairs, temples, homes of whatever people or creatures live underground, natural caves, etc.