Once upon a time when we played Final Fantasy 7, my brother would style his hair like Cloud's hair, give or take. One of our recurring jokes was that Cloud's hair stood up like it was gelled because of Hair Materia (Cloud doesn't wear gel, apparently, as Final Fantasy: Advent Children shows). It makes sense because there's no way I could get hair like that.

Anyway, if you could invent or use a type of materia that's not found in the game, what would it be, and what would you use it for?

I would totally take the Hair Materia idea. I wouldn't have to style my hair again and I could probably use it to augment my hair whenever necessary.