Massively

That's just a mechanic -- what about things that are exclusively fantasy? Magic plays a very large role in any fantasy game because it can be used to get around so many different problems such as permanent death (or perma-death). Personally I think magic has to go out the door in a good sci-fi massively game. The hardest part of making a game set in a sci-fi universe is going to be the world-crafting without magic. It has to be compelling, sure. Then how grounded in reality should it stay? Eventually, when you leave too many things unexplained or say, "It just works." to the player, you're using the same magic crutch that fantasy MMOs rely upon. It won't be easy, but I think that a developer could figure out different methods around this problem by looking at today's technology and extrapolating it in a clever way. Cloning is an obvious solution to perma-death that games like EVE Online use. Another way would be to have a world where everyone experiences things through an android that they control from some safe location. If that android is killed, they would have to spend money on a new one.

What it really comes down to is creativity. Magic is easier to use lazily, but sci-fi makes a developer think harder -- it challenges them. If a clever, smart developer were to sit down and think about what they could do to make their sci-fi game both compelling and grounded with just a hint of the unknown they would have a pretty great game on their hands.

I mention the unknown not in relation to explaining game mechanics, but in relation to the world surrounding the player; the undiscovered. Exploration; that's another thing sci-fi developers should pay attention to as well -- the mysterious and unknown. Trekking out into the great unknown of any world is so very much a part of the fun in any massively game. The experience's quality is what is important here, not the experience itself. Every game has an unknown world to explore in some sense, but they're not all equally worth the exploration.

These are just a few ideas, but generally speaking it isn't impossible to take what parts of fantasy that work well and graft them into sci-fi. The folly that should be avoided is to simply re-skin a fantasy MMO with a sci-fi appearance. Thankfully, the massively genre is all about evolving from one generation of games to the next. What I hope happens with whatever sci-fi game to come, is one part revolution and two parts evolution. It never hurts to have one or two brand new ideas, but we can learn from fantasy's triumphs and mistakes.