Deus Ex MMO
Why. in the. HELL. Has this never crossed my mind before? Oh, wait, yeah, it's my favorite game combined with my least favorite genre (slightly below Rhythm-Based Gem-Matching JRPG-esque Golf Management Sims Featuring Panda Bears, so it's well down there). I'd love to see it done along the lines of Guild Wars, but with, at the very least, regular major story chapters where the world and story evolve based on player's choices and, of course, who wins. DX's hallmark, multiple paths in an objective based map, either solo PvE, co-op PvE, or PvP, would be butter. Give each battle a point, working your side towards a group, area, or faction goal, and reward it with everything from player XP and technology, to affecting PvP areas in your favor, to (eventually) giving your whole faction a boost. If you want to give high-level players something to do when they hit the cap, why not let them work for their whole 'side', rather than just raid time and again in hope of some Tier 6 drops? And when one side does get ahead, let the other(s) run counterintelligence to catch up, but make sure the reward's still worth it. Give a good risk/reward to missions in addition to multi-stage objectives and payouts (two players sneaking past a dozen in a high-sec facility pays out far better than smashing up a lightly defended guardpost in the city). And I could go on and on, but I'll rein in the page-long post I could easily make right here.
Curse you, Jerykk, for putting that idea in my head. Now I've gotta wait for (Eidos to go bankrupt*?) the it'll-be-not-quite-as-good-as-the-original-but-better-than-IW DX3 and hope that its multiplayer, if in existence, will even come close to the possibilities popping 'round my neurons right now.
* Financially, not morally or creatively. Past the expiration date for those.