The defining Sci-Fi MMO is already here. The future of EVE Online may see it becoming the standard against which all future sci-fi MMO's will be measured. imagine a universe where everything from giant fleet battles in space happen concurrently with ground assaults on the (Azeroth sized) planet they are above. Close quarters man to man combat in defence of a space station. CCP have seen this universe and they will make it happen.

Leaving EVE Online aside for a moment though, why has Fantasy been prevalent? One reason and one reason alone, everyone knows what a fantasy game should be. There should be swords, magic, monsters, dragons and quests for items of great power. Everyone knows this, it is ubiquitous. Sci-Fi on the other hand has no central image which everyone knows, as a genre it ranges from space opera to post-apocalyptic and has as many different formats as there are sci-fi writers. until there is an image of Sci-Fi which is as widely accepted and iconic as classic 'swords and sorcery' fantasy there won't be a world dominating Sci-Fi MMO in the same league as World of Warcraft.