Sadly, the only MMOG I can think of that isn't fantasy or sci-fi is City of Heroes, but that's kind of sci-fi too. There's also WWII Online; about which the less said, the better. Generally, as soon as you take MMOGs to the real world you complicate matters so much it isn't even funny. Where is it set? Are you going to model geographical locations accurately? Will there be product advertising? Everyone wants product advertising. Is that a Dell PC in the corner?
But quite frankly, that's no fun. So what would you design, if fantasy and sci-fi were excluded? For me, 1930's zombie invasion. Who doesn't love zombies? Now imagine if they wore bowler hats.