With the way WOW has scaled its mobs, they could easily change individual difficulty without having to assign separate servers. Option 1 (Easy/Casual): Mobs have 50% Damage / 50% Hitpoints, Option 2 (Normal): 75% Damage / 75% Hitpoints. Option 3 (Hard): 100/100, Option 4 (Hardcore): 200/200. If permadeath was an option, that should probably be on a separate server. Loot could even scale. Honestly, the best thing to happen to WOW in years is the Mythic Dungeons scaling like Greater Rifts. They are brutally challenging above M+4 with the gear level available right now. Random secondary modifiers on mobs also adds a lot of challenge (mobs have higher hitpoints, or mobs drop pools on the ground, or enrage at 30%, or twice as many trash mobs).