I was playing earlier and i thought to myself what kind of materia would make the game a little more interesting so here are some ideas I came up with (if someone has already said something about this then ignore):

Skill specific materia: Materia geared toward a specific skill, for example a materia that makes a SMN have an extra stack of Aetherflows.

Farming materia: so this one might be both good and bad but anyways a materia that gives a percentage toward gil picked up.

Companion assistance materia: basically something other than the SP that you use to make your chocobo stronger. For example a materia that increase a companions strengths depending on stance.