Halivar said:
Huge swamp. No civilization. I'm thinking a black dragon. Or a green one. Or both. Maybe they're at war, and using humanoids as proxies. The players inadvertently stumble into it. But our dragons are smart, see? They know the PC's are more powerful than ten-times their number in humanoid yard-trash. So they both attempt to "persuade" the PC's to aid their cause against their mortal enemy (with a carrot and stick approach, "I'll perhaps give you treasure and perhaps not eat you."), with the desired end result of both ruling the entire swamp and eating the party.
Ditto. Black and green dragons, and large numbers of dragonspawn, half-dragons, etc. Barbarian tirbes led by dragon shaman holy men.
To make this interesting have what appears to be a vast uncivilized wasteland turn out ot be rife with life and society, just different than what the PCs are used to.
Forestlord elves, with green dragon blood, live in a hidden metropolis made of living buildings, plants shaped over generations to serve as buildings.
Dryad oligarchies rule over fey kingdoms and their druid allies.
Ruined cities, destroyed in the war between the dragons and consumed by the swamp their war created, are filled with restless undead. Maybe even a city of ghosts that continues to function as if it were real via Ghostwalk campaign rules.
Gateways to elemental planes of plants and water provide swift passage for those willing to use them as dangerous short cuts.
Lost expiditions from major universities beg the PCs for help finding a way home.
Awakened ancient cypruss groves, all a single entity, with levels in sorcerer can be a deadly trap or can aid helpful PCs who take the time to learn to communicate.
A crashed mindflayer astral ship is filled with the desperate and starving remains of the crew and an escaped band of thralls that war with them.
Big empty lands have lots and lots to do in them, and a swamp can limit how much one group knows about another.