After seeing some other artists posting fanfic concepts on this site, I decided to try it out myself.
The story's name is Cross Crisis. It's based on Dissidia: Final Fantasy NT, though with some differences. For one thing, Materia and Spiritus are alternate versions of the game's characters, in that they are actual gods instead of perfect Manikins. There are also no Manikins as enemies. Certain villains will be able to summon monsters or their personal mooks from their own worlds, though many of them will run wild with no control. Characters also retain all memories from their home worlds and lives instead of starting out as blank slates.
B