If you want to homebrew a setting like FFCC, I'd suggest looking into Command & Conquer. Might sound weird since they have totally different tones and genres, until you realize that miasma and Tiberium are the same thing. Compare and contrast:
Both of them (Tiberium and miasma) originated from a meteor hitting the world. In Tiberium's case, these meteors were sent by Scrin (aliens) to terraform Earth into a planet livable for them prior to settling it, since they need it to survive. In FFCC, the meteor is a parasite which spreads miasma, and is protected by a demon called Raem that feeds off of the suffering which the miasma causes. I like C&C's approach more, the meteor is a calculated thing by the villain, while Raem seems like an opportunist and the meteor itself is random.
Both spread around a lot. In C&C, civilization is mostly restricted to ever-shrinking Blue Zones, where the privileged and wealthy people of the world live, most of the world is compromised Yellow Zones, where the poor and oppressed live, or unlivable Red Zones. In FFCC, towns are built around crystals which keep miasma from overtaking them, proved they are fed myrrh, which must actively be sought out. I like both of these: having to seek out myrrh is a great classic adventure hook, but C&C's divisions make a more interesting world.
Both poison the world. Tiberium will rapidly kill a person if it contacts their skin, as it grows and covers them, and converts their body to more Tiberium like some sort of cancer. I'm not sure what miasma does, perhaps it suffocates the breather? It's a poisonous gas. Related, both of them cause mutations. Apparently, monsters in FFCC are animals or people mutated by miasma, while in C&C you have classic dumb sci-fi brute mutants. Personally, I like to go my own way here, and make up all sorts of mutations that could befall someone caught in the ether.