The basic idea is panfandom Sentai/PR game. Characters are brought in, players can pick a unique weapon and power, then describe a mech for them. They can choose a color or be assigned one. Mods would assign player mechs as "arm, leg, or torso" (for example) pieces and any complete set can combine to make a combined mech. Mods would assign it so no part is in high demand.

Possibly AU, though plot could be bad guys are the ones that kidnapped the characters from their canons and when they win they can all go home. Good guys are supported by npc organization that supports them, gave them the power and mechs, and wants to be helpful overall and defeat the evil bad guys from depowering everyone in every universe making them ripe for conquering.

I could not run, no time, this but I could pump out tons of ideas for it if people are interested.