5. Massively multiplayer war games
This is one of the genres that, I think, has not been created as of yet. I imagine either an RTS or an FPS (or a combination of both) experience in a massively multiplayer universe that takes place in a war of the worlds.
RTS style: Every new player would obtain a planet or a country and start his campaign to become the universe’s or planet’s leader from there. Kind of like current browser games work.
Like in Supreme Commander, you command all of your forces at once and can zoom out so that you can watch everything. It would be hard to control everything, but that would be the most interesting thing. You build units, control your forces, attack enemies, defend your territory, forge alliances with other players… Your progress is persistent of course. Your units will stay where you order them before you log out. A problem would be the defense of your territory while you are away but there certainly are possibilities to make things fair. Every player is essentially on the same map but you can only see what your reconnaissance units discovered, what your intelligence agency brought up or what your allies told you. There’s always a fight for resources and expansion is dangerous without proper planning… A whole concept would have to be far more exhaustive than this but it is one of my dream games and exists solely in my head.
To sum up, I would like to see a real time strategy massively multiplayer game which is a hybrid of Civilization, Supreme Commander, Risk and one of the thousands of browser games out there.
FPS style: You can choose to fight in one of several armies that all try to achieve world domination. You can fight in trenches with thousands of others at the same time. The scale would be like a real war. Everything happens at once. While you are running for cover from artillery fire, several hundred miles north, your fellow soldiers could advance further into enemy territory, taking over cities and important supply roads, for example. Front lines would shift constantly. You could, of course, travel there, although it would take very long on foot and you would probably be shot for disobeying orders 
There would also be planes and ships a team can control.
To sum up, it would be a recreation of a world war (possibly set in the future) and you would be a soldier in it. You would fight with thousands of others at the same time to create a dynamic experience.
Even better would be a combination of both. Some players would be the leaders and would lead the country RTS style. The troops would be controlled by other players FPS style and the outcome depends both on the skills of the commander and the skills of the soldiers. (How you give orders and how they must be followed is an issue, of course, but it can certainly be resolved)
Such full scale war games are of course utopian. They are not even on the horizon and maybe won’t even be created, ever. However, these are games that would be enormously fun, in my opinion.
