Well, MMOs are my favorite to design. Sandparks, as it says in my signature. Which is basically an RPG/sim hybrid, also a thing I really like in single player games. I play at least 70% of the game genres that exist though, and at least half of those would be fun and interesting to design.
The MMO Preferences Survey I recently created and got 114 respondents for had two pairs of questions about story genre and setting, I'll paste some of the data here for you. The data is % of all respondents who might enjoy a game with each setting. Non-Earth fantasy was the most popular setting, though almost all settings were potentially likable to at least half of players surveyed. War was the most popular atmosphere (vexing to me since I don't like it, and my favorite, Cheerful, was one of the least popular). I didn't get to look at the write-in results for this question yet.
Settings you might like in an MMO:Modern Urban/Suburban 49% 56 Post-Apocalypse (urban ruins and scavenging, zombies are optional) 67% 76 Archeological (tombs, fantasy ruins, alien ruins, anything Myst-like) 63% 72 Realistic Historical 38% 43 Historical Fantasy 64% 73 Non-Earth Fantasy 82% 93 Steampunk Or Other Historical Science Fantasy 66% 75 Non-Earth Science-Fantasy 74% 84 Near-Future Science Fiction Involving Earth Or Humans 61% 69 Far Future Or Non-Earth Science Fiction 73% 83 School (could be a realistic school, a magical school, a ninja school, a space school, etc.) 17% 19 Uninhabited Island or Planet (this would only be a partial MMO setting, since players would have to have some kind of city or other place to interact.) 49% 56
Atmosphere you might enjoy in an MMO:War - almost all of the story is about a war between two or three factions, and honor and revenge are probably themes. 76% 87 Politics, Blackmail, and Backstabbery - This is a story about secret maneuverings and manipulations, maybe brainwashing; probably some psychodrama in there. 68% 78 Gritty - the story is mostly about crime, drugs, supervillains, oppressive governments or corporations, enslavement, pollution, or other dark topic. 58% 66 Cheerful - the story is mostly about humor, romance, friendship, cleverness, teamwork, or wondrous adventure. 37% 42 Philosophical Or Surreal - There are probably references to Nietzsche, Kant, Locke, and Hobbes in there somewhere. 52% 59 Oldschool Heroism - whether about knights in shining armor or superheroes in capes or white-robed wizards, this is a story where the difference between good and evil is obvious and there are no gray areas. 69% 79 Other 10% 11