Well the first thing I would advise is not to make an MMORPG if there is another game type that will fit your universe or story better. However, since it doesn't appear that you have a story, premise, or world in mind already this strategy probably won't work. It seems you work opposite of how I do so it's difficult to give you advice on the subject.

I usually come up with a fictional universe (fantasy or sci-fi) and story first, then think of what game type would be best suited to let the player experience it. I rarely have a game-type in mind then try to decide on the genre, world, characters, story, etc. afterward.

However, as a blind taste test on the subject I happen to prefer sci-fi to fantasy slightly more, but only if done right. Usually, I try to avoid reading books, watching movies, or playing games that are based on Earth's future unless they are presented very well. This is simply do to the fact that as soon as you relate your story to Earth and its history your hands are instantly tied in a lot of respects. It drastically limits what is possible in your story's future and it makes the story lose any timeless quality it could have possessed.

Take Back to the Future II for example. By the year 2015, which is only eight years away now, it will lose any plausibility it may have originally maintained with the audience. The Terminator movies are another great example of movies that are going to eventually become obsolete. I for one don't want to be watching a movie that's supposed to be taking place now, but portrays the world as being enslaved by a distant alien race. It'd be difficult for me to suspend disbelief. This doesn't mean that it can't be done, it's just much more difficult and requires a significant amount of thought.

Now, if your story takes place in the year 2500 your not going to have this issue too much, but I would still prefer a fictional universe that is detached from the existence we know that includes Earth. Events in Star Wars for example are completely independent of Earth's history so the universe had tremendous freedom in what could be done.

Fantasy could be great too, but don’t create another Tolkien based clone. Introduce new characters, races, lore, etc. There’s already thousands of indie teams attempting to make these types of games and competition is fierce. Unless you have a unique skill advantage, piece of technology, or a brilliantly original twist to the genre I would not suggest going this route. I love fantasy games, but I only have time for maybe 1 or 2 a year. If only indie teams realized that most players feel the same.

I would prefer futuristic because the market is saturated with fantasy. I want to play something a little more unique. But if it's going to be one of those futuristic MMOs where you stand still and fire automatically at creatures, forget it. I wouldn't want to play it at all.

I'd play anything that had "Fallout" or "StarCraft" in it. I'm a mindless consumer automaton when it comes to those franchises.

With regards to fantasy games, I'll play pretty much anything that isn't wantonly stupid. I don't find stereotypical creatures/classes/equipment offensive, although including a good selection of under-represented elements will pique my attention.

If you base your creatures upon actual myths and legends rather than only the archetypes established since Tolkien, I'll be particularly impressed. If I can distract an attacking vampire by sprinkling grains of rice on the floor, that'll have me telling everyone about your game.

With futuristic games, I have stricter demands. Specifically, I want a futuristic game to be set in a feasibly possible future of the Universe I currently inhabit. As such, I don't want psychics or zombies, and I don't want mutants with superpowers. If it's very far in the future, I want to know why the human race hasn't converted the entire mass of the Solar System into a computer and uploaded themselves into it.