Dunno if anyone else here has played Blade and Soul yet but their open 24-man raids function a lot like I expected the Diadem to. An open map filled with trash and bosses; killing a boss has a chance to spawn a rarer, harder boss in its place with better loot. After killing 9 bosses the 24-man zone boss spawns that everyone converges on to kill. It has a lot more structure to it compared to the Diadem but is ultimately very similar. I would be happy if we had something more along those lines.