Here's a few off the top of my head.
-Bring back combat leves and make them relevant (as much as people hate the animus books, they sure got people doing content in the open world)
-Utilize fates better (make them part of a relic step or whatever, doesn't need to be some OG atma nonsense, just make it like 10 fates in a zone, anything to get you to actually go into the new zones)
-Get back to doing 2 or 3 dungeons per patch
-Give variant its own roulette (the roulette version can pick a random route for you so you don't just always end up going for the easiest one) and add better rewards to it (more glams, mounts)
-Make the 24 man gear the same item level as savage and upgraded tomestone gear (there hasn't been a reason to farm the 24 man gear for anything but glamour since they added the upgradable crafted gear)
-Experiment more with treasure dungeons (the kill a bunch of mobs for a random chest format is so tired at this point and none of the drops are worth anything, make them more like real dungeons or something and add way more rewards to them)
-Add more solo or small group instanced content (like the masked carnivale for every job or something like guildhests that would be harder and with actual rewards)
-ADD SOMETHING TO THE GODDAMN ACHIEVEMENT TOKEN VENDOR (what was the point of the whole token system when they do literally nothing with it, at least add the rare dyes in there or something so people can actually use the damn things)
-Add super expensive mounts/glamour outfits to the Gold Saucer (when GS came out everyone wanted that Fenrir and it was a lot of work to get it, they need to add some really high cost items to make MGP worth farming again)
-Utilize the housing areas for events/content (this is more a personal pet peeve but the housing areas just feel so barren and lifeless, it was amazing back when we had seasonal events in them)
-Make a new Eureka type zone (Eureka now after all the patches to it is great content and the NM trains in Anemos were some of the best times I've had in the game. It doesn't even have to be tied to a relic. It doesn't even have to be multiple zones, just make one big one that gradually opens up areas or something)