Here's a bit of speculation:

The bonuses we have on equipment now can be +, - and %. Based on this we have a good idea of the types of bonuses the materia can give.

In most other games using a similar gem/slot system there are tiers to gems, usually with the ability to combine multiple lower tier gems to make a single higher tier gem.

Some games like Monster Hunter have stat gems as well as gems that give passive abilities. I don't foresee this happening though as that is an integral system for that game and doesn't exist in this game at this point.

In FFVII there was a materia growth system. It would be interesting if we saw that make a comeback in some form for the materia.