They need to just go all in and make diadem more like XI's Abyssea. There are so many changes they could make that would improve Diadem and make it interesting, which is what made abyssea so much fun to be in. Simple things like
  • Locking enemy rewards to only the alliance that claims (incentivizes groups to spread out to all parts of Diadem and not pack-rat the entire instance onto the same island)
  • Diverse chests with different drops
  • Simple weakness system
  • Force spawn S ranks using key items, not random points generated by mindless killing
  • Ability to extend your alliance's time in Diadem

And so much more. If anyone doesn't know anything about Abyssea, just check out the wiki and see how many interesting and different things you can do there. http://ffxiclopedia.wikia.com/wiki/Category:Abyssea