How to make gear progression non-linear

[lackofwords]

There's really no reason to create new skills when you can just create equippable items that alter the functionality of your current ones enough to make them feel like new abilities altogether. If endgame Materia functioned that way, it would essentially equate to every new drop becoming a new skill for your character, or a possible alteration to your rotation.

What exactly do you have in mind that can alter a skill rotation?

Cause at the moment I can't help but think 'why items and not skills?'
Let's take a look at Cleric Stance, it swaps MND and INT stats around, basically WHM switching to combat while not healing with a small downtime to switch out of Cleric Stance.
This does not necessarily alter skill rotation, but enhances your attack immensely. Such as the same effect as if someone were to change into higher damage dealing gear to perform an attack, before switching back to another gear set.

Now let's say there's a fictional materia that adds HP leech according to the number of enemies you hit, you're trading this off with another materia that could increase your class's base stat.
So let's say this is for a tank, and this materia is only useful while facing large numbers of enemies, since the HP recover rate would be higher than the trade-off of lower defense. Thus, it wouldn't be as useful when tanking a single boss target.
So, why make it an item that you have to carry around, and swap into when the situation occurs?

Why not make this into an ability like Cleric Stance?
It'll serve the same purpose without taking up inventory space, nor the hassle of changing out of gear in between non-combat phases.
(not entirely different weapons though since that changes classes)

An even more interesting application for Materia that enhanced abilities would be Cross-Class abilities. There are plenty of abilities that are cross-class though completely useless on jobs because they lack a combo ability or are eclipsed by your natural skills. This type of system could open up all sorts of possibilities concerning that.

I'm not seeing what you're imagining here. Like, would it allow the cross-class abilities to combo with something else?

Either way, bottom-line, as a materia it is still a slot trade-off, which in turn will make a player carry different sets of equipment around if the trade-off is distinct enough.
And we already have two sets of gear we need to carry, PvP and non-PvP for each class.

The only way I could imagine it working, as I think you would want it, is if the materia was actually like FFVII materia system, where you chain additional abilities and skills onto the weapon as well as support materia that enhances the those abilities and skills. Path of Exile also does something similar. The character has only passive abilities, while all the active abilities are in the gems themselves.