Come up with new materia and discribe how it works. remember there are only 5 different kinds of materia. Summon, Magic, Command, Independent, and support. here are a few examples,
Magic - Titled "Supreme" and can learn Holy, Meteor, and supernova.
Summon - Titled "Atlas" and the attack is a man with the world on his back throwing the planet into a blackhole and then spitting it back out into space. The attack would do more then 9999 damage.
Command - Titled "Pray" which in FF lets you heal some HP by chance and uses no MP.
Independent - Three titled "Break HP" "break MP" and
break Damage" which means HP MP or damage can go over 9999.
Support - Titled "Aura" with this linked to anything that damages an enemy. The action will boost your limit gauge.
Have fun making your own and yes I took all the popular ones...