Apologies for a kind of double post, but I thought this was an interesting enough idea that it was worth a thread of it's own rather than getting buried under another topic's rantings:
TLDR: How could we have more gear variety in FFXIV?
Rather than get blinded by how FFXI handled things, let's look a bit further back to the days of Everquest. The game was primarily horizontal progression with mild levels of verticality between expansions and tiers. The overall quality of gear was noticeably better in late tier raids such as Vex Thal when compared to old endgame content such as VP or recent but entry level content such as the first planes in PoP.
However, most endgame zones had a few pieces of 'Halo' gear, items that were unique enough or good enough to stand above other options even 4 or 5 expansions down the line. As a Shaman raiding up until the mid point of PoP, I was still utterly inseparable from my VP Robe and Snare spear and overall wore a variety of bits of gear from a content right across the game ( https://eq.magelo.com/profile/9093 ).
So, how can FFXIV pull this off? I'd go with a variant on Kisa's approach but roll it into a form of set bonuses. Let's say I have a full set of Sigma gear, consider having a set bonus that can be unlocked by wearing 2 pieces of gear depending on where the gear is from. PvP gear could unlock something like a cast time reduction on Stone. Crafted could unlock some extra VIT. Hunt gear could unlock a movement speed bonus. There's plenty of ways to add in flavour to this and it's simple enough to roll extra bonus stats in with the set bonus so that you're not destroying your main stats for the sake of these sorts of bonuses. When odd numbered patches come along with new gear, it's easy enough to bump up the bonuses slightly on the new stuff to give the raiders and min maxers a fresh carrot to chase.
Simple no?
*edit*
To clarify since the original post was a bit rushed and not as clear as it could have been.
My idea is to actually have the set bonus based off the main raiding sets aka Tome and Savage gear.
This would offer a set bonus when used in tandem with other sets within the same tier as well as offering a stat boost to bring the lower ilvl piece up to par with the main set.
So for example:
Lets take the Diamond set of healing.
It could have the following set bonus as a very rough example:
0.5 second cast time reduction on Stone IV when used with 2 pieces of PvP gear.
+100 VIT when used with 2 pieces of crafted gear.
+10% Movement speed when used with Hunt gear.
This could be further played along by adding a slightly juicier bonus with odd numbered patch content (Basically, extra crafted and 24 man stuff).
The idea isn't to make people not want to upgrade their gear, but rather reward players for earning a wider variety of equipment (and thus dabbling in more content that they otherwise might not. The crafted bonus will be king during progression, whereas it'd be easy enough to include interesting perks on other sets to take over as people get geared and don't need the VIT anymore.
Is it a perfect example of horizontal progression? No. But it spices up a recipe that's been getting awfully stale over the last few years.