There is a pretty hefty portion of old content in ffxi that is extremely outdated, and has two factors in its uselessness.
#1: The rewards from a lot of old content are worthless, but once were valuable.
#2: This content takes a LOT of effort to utilize, and is fun for some players, yet because of point #1 no one bothers to even touch the content.
The issue really just simply lies in point #1. There is a massive amount of fun, old ffxi content that is worthless to do because players get essentially 0 reward from it.
If square enix really intends to 'finish' ffxi, before they can consider that option they really should at bare minimum take all this old content I shal llist below, and quite simply just add a handful of rewards to the content (literally just replace old useless items) with new drops that have some use/value.
That is really all it should take. In other words, the stick is already in place for all of this content, much of it was very worthwhile doing and very fun for players to work at. There is no carrot on the stick anymore though, simply replacing the carrot on the stick (via updating the rewards / loot tables to be relevant again) should add hundreds and hundreds of hours of viable play time to ffxi again.
Lets begin:
Gardening: This is currently an essentially useless past time that used to be very rewarding. Please add a handful of valuable materials that can be used for 100~110 crafting to gardening. Even just a couple being added should be enough to make gardening useful again.
Chocobo Digging: Used to be very very profitable digging for elemental ores, but not anymore because of abyssea's drop rates on ele ores. Please just add a couple of relevant items to the zones for chocobo digging. This is literally all it would take to make chocobo digging worthwhile.
At this point, a HUGE amount of old content suddenly becomes worth doing again in a cascade, as much of the old content in ffxi hinges off of other content, observe:
If gardening is worth doing, chocobo hot and cold becomes worth doing as its one of the best sources of getting arcane/wooden flowerpots, which are the best pots for gardening. Furthermore the very old arcane machines around vanadiel become worthwhile running to do their quests as they also reward arcane flowerpots.
If arcane flowerpots become valuable again, then all of the above is worth doing.
If chocobo hot and cold / digging is worth doing, then chocobo racing becomes very worth doing too as it is the only way to get chocobo coats that improve your ability to do hot and cold and digging.
If chocobo racing is worth doing, chocobo raising is mandatory too of course.
Moving on:
An Explorer's Footsteps
This quest was once the way to get the map of crawler's nest. Now that it isn't, its not worth doing. Please add a nice fat reward for players who collect all 18 tablets, to give incentive to go explore the nooks and crannies of this world you made. Maybe something as simple as a special title, or a pile of gil.
Black Belt
This belt is a mountain of work to get, and a sign of a serious monk. Please either follow up on your promise you made to add another tier of belt (maybe a reforge quest to +1 it), or update the stats on black belt for it to be a usable belt.
A belt with martials arts, double attack, triple attack, quadruple attack, in some combination, for example, is miles more valuable then 12% haste.
For example: Dmg-5%, martial arts+20, DA+1% TA+1% QA+1% is a very solid belt. Current black belt is useless despite the hours and hours and hours and hours of work it takes to get. Please fix this.
These are a handful of the large scale, highly time consuming events of ffxi that are still fun and interesting to do HOWEVER since their rewards are... just not rewarding at all, no one wants to do them.
The tiny, simply act of updating rewards of all these events would jack up hours worth spending on ffxi by a large margin, and would make players happy and also add a lot more longevity to ffxi.