I just thought probably worst possible genre for mmorpgs. Underwater sea mmorpg. You fight the enemies with watertrident and the enemies would be sharks, fishs, clamps, everything underwater. Tho the cool feature would be this could be totally something like space-fighter mmo just underwater, you can move every direction, not just back, forward, left and right. But seriously something like space fighting mmo would be cool. Where one needs tactics, and like strategy to drive enemies to mine-fields etc. The range of the weapons just would needed to be somewhat gapped. Like not that you see one enemy 4 lightyears away and you could just blast it off.
Also not so much of genre, but idea what i would once like to see in mmo. In ancient game like chrono trigger there were combo skills, where you needed like 2 to 3 guys perform a single powerful attack. This could be made use somewhat like usage of weapons. Like you could dual wield a fire staff and ice staff then make spells that use both of the elements. Or maybe you could like cast fire-spell on party member and next skill is going to use the fire spells power that was casted upon. For example you cast fire buff on monk, monk casts next shield on all players ---> it becomes fire-shield.