How to make gear progression non-linear

[lackofwords]

This actually isn't a case of power creep though because newer content isn't going to ruin the balancing of the game, it's just going to render old content no longer optimal. Which is pretty much how it's supposed to be.

The only thing non-linear gear progression really adds though is a ton of replay value and alternate playstyles.

Isn't that essentially power creeping for mmo? Rendering old content (in this case gear) obsolete...
Also isn't mmo suppose supply a ton of replay value?...

You could argue that the ilvl ranges are stepping stones to the next set, but even then there aren't that many steps to take. Random rewarded gear -> AF set -> DF set -> choice of ilvl 90 items unless if you want to play the other fights.
*edit: They are adding or hinting at crafted gear for patch 2.2 will be better than dropped gear, which would essentially be what you're suggesting. http://forum.square-enix.com/ffxiv/t...-2.2-confirmed
As for replay value, there are people who would only finish the main story and end the game there, but there will also be people who wants to play the game as a different role or different build.

Making materia drop in dungeons or purchased via tomes would surely increase the supply and demand of rare materia, and also increase the grind on those same dungeons.
I'm not sure about you, but I'm already sick of the running the dungeons, trials, and guildhest for tomes.

Even the just novelty of learning new boss fight patterns and mechanics is not overcoming the repetitiveness of skill rotation for me.
Playing a new class makes the player learn a new role, as well as a new class skill rotation they would have to use, which in turns adds to replay value in the end.
However, for me what was a breath of fresh air was the addition of PvP, which also introduced new abilities. The combat there forced me to think of new ways to approach new situations.

As it is now, gear only adds a boost or multiplier to your output, but does not change the way you approach your fights.
Gear swapping would add an additional micro action the player needs to keep track of, but at the core functionally of it would serve the same purpose of abilities and skills.
E.G. Cleric Stance, swaps MND and INT, and is not affected by GCD.
Why just add new abilities instead of the cumbersome load of holding multiple gear sets around?
Could make trials to drop random scrolls that teaches new skills, and limited how many you can equip at once. Save everyone the hassle of an inventory nightmare again, while still providing the player with option to micro-manage battle stances or whatever non-sense that comes along with gear swapping.