Back in the day when the genre was new, people were fascinated by the potential of virtual worlds and virtual societies. Social scientists did online studies on player behavior and the interactions people had online, on spontaneous self-governance coming into existence, on how communities formed and developed, and many other similar topics. That potential was never fulfilled.
Today - some twenty years later - the MMORPG has become a genre of checking off boxes and making numbers go up, along a linear way as laid out by the developers for you. Apart from PvP and maybe some forced grouping, most games would play absolutely identical mechanically, if you were playing all alone on your own private server. You'd do the same quests, fight the same enemies, get the same loot. All the other players you get to meet online - they don't actually influence the game mechanics at all.
You play next to each other. Not actually with each other.
I'd like to see a game, where the sum of players (and their interactions) are greater than just the sum of it's parts. A game with a virtual economy, a virtual society, etc. - that advance and evolve in a player-driven fashion. A simulated game world that dynamically adapts. Some glimpses of this sort of thing can be seen in games like EvE. Old games (pre-WoW) like UO and SWG had some of that magic as well - but were marred by limitations of the technology of the day. This kind of stuff has evolved very, very little since then.
I would assume that with today's technology we should be able to get a lot closer to fulfilling that potential.