Being a 10-year old game, the amount of armor designs that have entered FFXI is considerable and impressive in its own right. At the same time, the game suffered from players being funneled towards specific gear, which leads to them looking the same. This goes as far back as the days of the scorpion harness/haubergeon+optical hat army.
As someone who has enjoyed the armor designs of XI, it does bother me a bit to see that so many have vanished as the gear trends changed. I get that people always go for the gear with the best stats (as they should), but I've also derived some fun from aesthetics.
With that said, I'm not sure which of the existing approaches to appearance control would actually fit FFXI, hence the point of this OP. I can't specifically suggest one simply because I don't know what would work and what would not. As such, I've chosen to present the ones I am familiar with for the sake of discussion.
1) Appearance enchants: The Secret World currently uses this system. You turn an item into an "enchant" that, when used on a currently-equipped item, gives it that item's appearance. An FFXI example would be turning a dalmatica into an enchant and then applying it to a Praeco Doublet, replacing the doublet's appearance with the Dalmatica's but retaining the stats.
2) Item modification: The Old Republic uses this one. Items can be modified with parts that correspond to that item's stats. While the item itself can be kept, progression instead focuses on the item parts that determine the stats. An FFXI example of this would be a Haubergeon having four parts, each determining stuff like +STR, +DEX, +Acc and so on. A suggestion I made a while back also used this system as inspiration.
3) Vanity slots: Lord of the Rings Online uses this one, and has been suggested by Buttons on the FFXIV forums. You unlock something not unlike equipment slots, and placing gear those slots determines your appearance.
4) Appearance lock: FFXI actually has this one, but only available for adventuring fellows.
5) Transmogrification: WoW uses this one. You pay an NPC an amount of money (that scales with the item's level reqs and relative value) to change the appearance of an item with that of another.
Aside from the expected "I'm better than you for not caring about my character's appearance!" and "lul the devs shouldnt waste time on this when they should do X", my two primary reasons for mentioning this are that the game has hit a point where there are a lot of unused art assets. The other being that appearance has repeatedly suffered to the point we expect the best gear to look like...well....crap.
Anyway, that's that. Questions, comments, death threats and so on are welcome.
PS: I purposely left out how appearance control should interact with gear swaps simply because each of the systems I mentioned would work differently with swaps. #3 and #4 would nullify the visual aspect of swaps (much to my joy), while #5 may make the whole thing a little more expensive if you choose to transmog EVERY swap piece.
PPS: there're obvious things that haven't been mentioned, like job restrictions per piece of equipment and such. Determining whether something like this is worthwhile comes before that sort of thing, though.
PPPS: there are positive side effects to this sort of thing (like it encouraging people to do older content), but again, that's for later.