I'm just going to premptively spoiler warning this whole thread.
I got to thinking - I'd like to try to play an outpost as if I'm managing 7 dwaves sent through a portal in Hidden Fun Stuff to confront the demon world head-on. Basically, dwarven hell fort! My thoughts so far,
Civs:
Goblins replaced by Frog Demons,
-- replace Trolls with some terrifying large demon thing
Humans replaced by Tentacle Demons, made hostile,
Elves removed
Dwaven traders function as normal
Wild predators replaced in all biomes by Spirits of Fire
(obviously there is a place here to add tons of custom demons, but I want the "canon" DF demons to be the most common/prominent)
Wildlife:
Initially, probably just removed to save effort/time.
Carp left in, and buffed, and allowed in all biomes. Obviously.
Flora:
Most crops are removed
Plump helmets grow, but much more slowly and with lower yeild
Native plants are mostly useless / poisonous, but maybe leave a surface plant that can be brewed. Maybe just leave in the current EVIL area surface plants?
Trees are removed, except for tower caps and that one evil tree.
Megabeasts:
Current megabeasts are all too... neutral. Will need at least one custom demon mega beast. Maybe super versions (Elder?) Frog, Tentacle, and Fire varieties.
Obviously, this mod will have the greatest effect if you start a fort in a hot, terrifying sand desert or rocky waste. The challenge, I think, would be to survive long enough to kill an Elder demon, thus accomplishing your mission on the infernal plain.
Just a wild idea, that I think is mostly implementable. I thought I would open it up to the community to see what delicious embellishments the community would come up with.
-Erom