Although some of my ideas I feel pretty confident about, armor I do not. What armor should actually do in the game is still kind of a mystery for me but I have some inklings of what it could be!
DO's & DON'TS OF ARMOR There are a few things I know that I DON'T want to do with armor though so I am starting there. For one, I don't want to pigeon hole people into only being able to select one type of armor. One way this is done is if you want to be a wizard for example. To be a wizard, you must or can only wear wizard cloaks, must have pointy hat, must carry staff. Often a character can only wear certain types like this or equipping a set gives you a bonus benefit that makes NOT wearing the full set preposterous.
The second thing is over complicating things. Like weapons, I could slap a bunch of keywords on them and maybe some special abilities to and call it a day. But something I run into board games often is actually remembering all the moving parts. I am sure anyone can recall playing a game and realizing when its way to late that you forgot to apply poison or added some bonus to a dice roll that would have changed the outcome of a battle. It is one of my biggest gripes of complex games and have read many times others would agree. So without further ado, here is what I am proposing.

THE WIZARD ARMOR

Ok, should you be able to be straight wizard if you want? Hell yeah, wizards are cool. But instead of gatekeeping things to a class, it will be more about what you are willing to take into battle! The Cloak gives players a Green Materia slot, which is healing skills/spells/abilities. Then it gives a straight HP boost. No keywords to remember or special moves, very simple. The Derelict Hat gives players another green slot and a red, which is all your damage dealing spells/attacks/abilites and a 1HP boost. Powder Boost gives you no HP boosts but access to a lot of Materia slots, like a Wizard knowing a lot of spells kind of thing.

KNIGHT ARMOR

For this next set of armor pieces we have a switch in priorities. These heavily lean into rem Materia which is damage dealing, and since you have no access to green matera for healing, the HP increases are a bit higher.

SHADOW ARMOR

This armor gives player almost nothing but Orange Materia, which is used for moving fast, dodging attack and doing additional actions. You could definitely put everything into straight dodging attacks, but if not you could always put on a different Helmet with some red materia to mix things up.

MIXING ARMOR PIECES

This I think will give players are easy way to mixe armor pieces. You may want the Powder boots to gain access to more abilites, which gives no HP bonus so you would then combine it with maybe the Spekkio Hands to get that nice 5HP max HP bonus. Of course this will all depend on the weapon you have equipped as well which will help determine what Materia you can even take into battle.

The hardest part as far as making a build will be to determine what you want do, what resources to you have available to craft something you want, and what is worth crafting at that moment. I am really just going to make as many different materia as I can to give you a crap ton of options. Even if a skill is kind of sub-par it could easily become better if the crafting cost is cheap and maybe paired with the right materia combo. We will see how it goes!