I started drawing a campaign map a few weeks ago. I have no plans for an actual campaign, but just felt the need to do so. I draw a new map for every campaign I run, and one bad habit I get into is the compulsion to put some kind of blasted wasteland to the north or west of the land in which the characters will begin the game. Wastelands aren't bad in and of themselves, but drawing one on most of my maps is a crutch I need to get past.
So, on this map, I decided to have an anti-wasteland...instead of a place that has been rendered barren or blasted or uninhabitable by a disaster, I have added a place that is overly verdant. You could almost call it virulently fertile. The fauna and flora are mostly hostile to demihumankind, and they are unlike any of the flora or fauna found anywhere else in the campaign world...although this is a fantasy setting, the lifeforms in this verdant expanse still seem "unnatural" to this world's ecologies. Plants might have animal characteristics, be mobile, be covered in eyes, etc. Animals are bizarre hybridizations of mammal and insect, land and sea life, or simply take some shape completely unfamiliar to the campaign world.
The PCs can get involved in numerous ways: they might have to cross through the Verdant Expanse to reach an otherwise unreachable place. (Or perhaps they are pressed for time and do not have the luxury of a lengthier course that bypasses this area) Alchemists, wizards, and sages might pay handsomely for specimens (living or dead) from within.
There is also, of course, the possibility that the Verdant Expanse is growing, swallowing up the land around it in its alien fronds. If something is not found to stem the growth, civilized lands may be in danger in a few years...or months...
For that matter, the Expanse might have grown over some important structure the PCs need to enter or retrieve something from.
Druids and rangers find that the lifeforms from within the Expanse are resistant, perhaps immune, to their usual abilities to control/soothe/influence/command/summon plants and animals. Perhaps the Verdant expanse is home to alien druids of some unknown species, or simply human or demihuman druids who seem to have gone mad or feral.
The DM may wish to assign some kind of good qualities to the otherwise exceedingly dangerous swath of otherworldly growth. Maybe fruits or other edible objects from within can sustain a normal man far longer than ordinary food. Maybe any natural healing done within the Expanse is doubled. Perhaps the plants can be used to make potent medicines or poisons, curing that which previously could not be cured by anything short of magic.
This was equal parts inspired by the Chtorr books, which I have never actually read but intend to one day, the last level of the SNES game Xardion, a dream I once had about my garden getting monstrously out of control, and my desire to do something opposite of my usual Obligatory Fantasy Wasteland.