I, and many others have disappointed mostly by diadem's boring grind till you drop mentality and the broken battlers vs. gatherer's problem that seemed to exist since the beginning. I suggest a few things that could change people's perception how you view this content and make it worthwhile content you find fun.Please Keep in mind that these ideas are meant to work in conjunction with each other.
1. Make it so the instance is used at the same time by 24 people, that they spawn in the same location.Also make monsters permanently die so you can feel progression by clearing out each island.
2. Change the goals to be more difficult to accomplish. For example:
A: Kill 25 IV/III tier monsters. Reward: Guaranteed Platinum chest(With multiple highest level items)
B: Clear out 3 Islands Reward: 1 Platinum chest
C: Kill at least 5 Star Bosses Reward: 3 Platinum Chests
3. Remove the time limit altogether and make the goal all about clearing all the islands rather than grinding until the time limit is over. Instead put a time limit for the goals so you still need to accomplish things within a certain time, but won't get kicked if you don't.
4. The reason for the removal of the time limit is to accommodate gatherers, so gathering only becomes possible after you clear out a area or island, and a limited amount of nodes within very desirable materials appear for gatherers, since there is no time limit you can now just spend your time relaxing after raid and gather up all the nodes after you clean out of all the islands. No reason for battlers to stick around since they got what they wanted, but at the same time you get to do all the sweet gathering without having to worry about time limit or monsters screwing with your steez!.
No Pressure, just nice relaxing gathering with sweet vistas!
5. Add mega bosses in that can only be accessed after all the Set Star monsters give you their keys to unlock. Mega Bosses will have their own special area/island that will have mechanics like normal raid bosses and can top off the diadem with something awesome. Maybe their can be different islands that are locked up and you get a random mega boss every-time. Mega Bosses will Drop Rainbow chests(or w/e you want to be higher than platinum) That Can drop extremely rare materials not got anywhere else that can maybe make Very Good crafting gear for gatherer's/crafters, Tier Six Materia, rare minions, etc.
6.Make Flying Accessible from very beginning, its pointless to force players to run around and get all the aether thingies and its frustrating when people leave you or your friends behind because they weren't paying attention or lagging etc.
7. Make all monsters a bit stronger than they are since they all permanently die now, yes a lot of monsters in diadem already have decent size hp, however i think they should also become more dangerous so they feel more like raid monsters than just hp sponges.
I think all these changes combined would be a reasonable way for Square enix to take the current existing frame work and turn diadem into something truly awesome. We could keep playing even into future expansion with more areas being added with unique bosses in future updates, making it a solid part of endgame that keeps being updated instead of boring grind that is eventually forgotten about.