The number one complaint about FFXI from former players is that they wish it was still like 75 cap - to the point of making illegal private servers, even, which is terrible.
A lot of players - especially long-time players - have heaps of lvl 75 gear from over the years packed away somewhere, lots on additional characters now, thanks to the ability to send more and more things by delivery box. However, there's not really any productive way to use any of it.

EverQuest (the inspiration for FFXI) added a type of instanced content at the level 80 or 85 level cap wherein you select a lvl 51 vanilla class, (iirc, only one of each class per group, like Sheol-Gaol) and do a classic dungeon crawl to the OG dragons. The rewards were current; many best in slot.

The opportunities are endless: 75 cap ballista, 75 cap versions of content from Abyssea to the present (instances which drop newer, more relevant gear), new 75 cap seal/crest battlefields, like the EQ style ones, to clear the path to, and defeat, Fafnir within the time limit. The KI from winning would let you get the Nidhogg version; and so on with all the classic HNMs... Just to think of a few.

Besides bringing in players both new and old alike, the super extra bonus feature is that these battles will be too difficult to pull off in scaled-down gear; you're going to need a good party in good 75 cap gear; this means "OLD CONTENT" that so many people entirely disregard, and sits neglected, will now be RELEVANT AGAIN! Not just relevant, but critical. This will have the extra EXTRA added benefit of spreading the population out across the servers; instead of everyone being in Ra'Kaznar, Rabao, Mhaura, Jeuno, and Adoulin, there will be people running all the old content for the best 75 gear. It would also be a good idea to spread out the spawn points or NPCs for these instances amongst many zones, for this reason. It'll not only give so much great, but sadly overlooked content some much-needed attention, exposing the new generation of players to such great content, and allowing some of us old-timers to go back and fill in what we missed, or wax nostalgic.

You could even make a 75 cap "progression" server; launch it as pre-Zilart, but with the modern QOL features, and "release" each expansion/add-on in the order they came out, but in a compressed and evenly-spaced timeframe.

There is a huge untapped market of "oh, but 75.." people that it seems imperative to tap - the influx of revenue could even potentially bolster a second resurgence, nay, a Renaissance, bringing another expansion and/or higher battle content for the bleeding-edge players. Maybe.

In any case, I'm not seeing a downside.