I can see a few problems with this.
First, what do we define as classic, and how do we go about doing so?
Pre abby? Pre Toau? Wotg? What about Adoulin content that's a part of the lore? Is cutting out large chunks of lore ok?
Is level sync classic? what about Fellow NPC's? And quality of life improvements such as those listed below, or uncapping promathia missions?
Is blue mage classic? What about geomancer and rune fencer? Scholar and dancer?
Would ranger be god again like it used to be? Or samurai the hot topic?
One you'd be setting the server back to before the december update of 2009. (Which was when fields of valor was added)
You'd be cutting out A LOT of content by doing that, Adoulin would have to be completely removed as there's no way Lv 75 players, even alliances couldn't roam through that zone in one piece. Either that or Adoulin would have to be scaled down, and mobs radically adjusted on this server to match that of either a scaling area like the original zones, or a 75 only region like Sea, because people will want to play a pack they paid 30-40$ for.
You'd also remove part of fellow NPC adjustments, quality of life improvements like movement speed, teleporting, WS adjustments, TP changes, gearsets, spells, job abilities and traits, hp and mp adjustments, avatars, mog wardrobe, mog case, RoE, and numerous other things, including half of the porter moogle. I'm not sure if campaign skillups would be in or out, I'd have to look through all the updates to find when they stopped it. And obviously relics would become scarce again, and mythics would be... mythical.
You'd also have to patch, update, and create content specifically for a 75 capped environment, this would double the workload for all future expansions as well, and you'd run into the same problems in the end: People don't want to do the same thing forever (Hell people in my old shell skipped sky 90% of the time, as well as dyna zones they didn't need), so you'd have to spend a lot of time creating new things for this server to keep the population, however big or small, happy.
And to even do this, you'd have to write a lengthy program to go through some tens of thousands of accounts of potential users, and possibly hundreds of thousands of characters for this server and prune all data relating to any gear, zone, or quest above 75 or related to any expansion data, then setup server transfers for those accounts, because a lot of people aren't going to want to lose their characters and start all over again, esp those who left because of abby. (I know a lot who did) But you'd have to be careful, people won't be happy if 75 or lower gear is stored in mog bags that will be removed and they lose it.
You'd then have to ensure server rollbacks didn't effect the client, because some video cards might stop working if rolled back to the old days (windows 8 platforms likely would). I know my geforce 600 series card has crashing issues with FFXI and high shadows when songs or spells go off. And players would have to run 2 seperate FFXI directories, one for mainstream (if they choose to play it too, not everyone will goto this server, so keeping in touch with friends would be essential) and one for the classic. This would be hard for some people who run FFXI on older PC's with limited space, or like me who just have limited HDD space (I run FFXI on my SSD), and could be impossible for PS2 users (Which is most of japan), though the 360 likely wouldn't be too hard once you programmed all of this into the viewer.
I don't believe FFXI has a skeleton crew, and it maybe the most profitable game in their history, but I don't know if Square is going to hire extra devs to make this server.
Would they charge extra to being on this server? What if only some characters per Id are there, and some on the main servers, do you pay full price for both? And would this interfere with the mainstream servers production and updates?
Because a lot of people likely won't move, I'm not sure entirely, if I would use the classic or not, my time is a lot more limited then it was in 2003 when I could play all day after work, so being able to login and go do the things I need or want to with my partner and not seek group in jeuno for 4 weeks just to get from lv 45-46 (I was a taru monk at the time), or watching pickup group after group fail to be promathia, or dynamis wipes because people don't know wtf they're doing anymore, and new players in FFXI proper don't have the skill sets needed to run in the old days.
I know a lot of people miss the old days, but I think a lot of people also forget how hard those days were.
After all, the number one complaint from almost everyone was: It took so long to do/get anywhere in FFXI. We'd be removing everything that made that easier, both the grind, and the travel, and get right back to where we started, with people leaving because of that.
As great as the community was back then, and the socialization, some things are best left to the pages of history.
This could be one of them.

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