Honestly I want something massive and entirely non-linear. Oblivion meets Civilization meets the big battles at the end of HL2.
The protagonist is thrust in the middle of a chaotic world where different factions are at war. There will be actual front lines with battles actively raging, which you can choose if you want to go fight in or not. You could choose to jump in the middle and try to help gun down enemies, or maybe you could choose some other way to participate, or not. In other words, picture a semi-destroyed city where two groups are going at it, and you can choose to fight alongside a group using conventional warfare, or maybe lead smaller more covert expeditions, or any combination thereof. Your actions determine the fate of your group, and your role within it (general, president, special ops team leader, etc). Also imagine that there are a great many of these cities, in different locations, and under different circumstances. You would prioritize what it is you want to do in order to achieve the objectives that you determine. In other words, totally open ended with massive strategy, massive battles, fantastic AI, and endless outcomes.
This could be either multiplayer or single, depending. I can imagine both scenarios being amazing for different reasons.
I expect this sort of thing to debut in about 2015.