I kinda wish all jobs could use summons since it's a staple of the series. It would basically replace Limit Break and be the full size summon you can call out after charging party bar, with different effects depending on the summon used. Like, Carbuncle would be what tank LB was, a shield effect. Any player would have access to all of the summon types though regardless of their role, so it might be that a dps player uses the 3-bar charge on heal+revive, or a healer activates the single-target attack one.

If this game had a system of collecting summons, I would want a really diverse set like Carbuncle, Midgardsormr, Fenrir, Diabolos, Ultros, Hashmal, Phoenix, Bahamut, Mateus, Zalera, Gilgamesh, Bismarck, Susano, etc included. Their essences could be equipment basically and feed them crystals for stat growth like FF VIII junctions, with higher growth items being available first from high-end raids. Some of the current equipment slots that are uninteresting would have been better as appearance-only glam slots to save on character data, and belts just removed altogether.

So, reducing the number of equipment slots but putting a higher degree of customisability on the ones that remain - you'd have to do something cool with weapons too right? Make it possible to grind up any weapon, but the higher you raise them beyond their original stats, the amount of work it would take to increase them further would go up exponentially. You could grind a Byakko weapon to +20 eventually to hit the limit of the current patch, and if you wanted even use a special item available in the next raid tier to break the limit on it and keep raising it to the new IL limit of that patch, but it would be easier to just get a new weapon at that point. Just an option for grindy players.

Maybe let us convert certain weapons into an upgrade item which is used to upgrade your chosen main weapon? Like there would be a point to farming 300 Susano swords if you really wanted. Let us buy augments with capped tomes or something so we can add materia slots to items? So your Augmented Byakko Sword +10 would be IL 365 and have 3 slots. I'm not saying let people get uber if they have too much time; you can still artificially cap grinds so casual gearing methods cannot catch up to raid gear until certain items are available months later.