If I were going to make a game...
Well there are two concepts I've had so far.
The first one is similar to Infinity with its procedurally generated world/universe. However, instead of just producing one ball-tighteningly fantastic galaxy with static scenery, it can keep track of player interaction with it, and adjust itself accordingly. Thus, if you want to start a colony, you can. Want to blow that colony up? Go ahead. As a twist on MMOs, instead of having each and every event be a prescripted boredom fest, the players generate all the chaos and commotion. You can build up your stats, unlocking new abilities--like 'build installation' or 'ransack village.' Each action of this nature creates an event other players can respond to, and its success or failure will alter the world persistently. I see a bunch of people yelling 'EVE, EVE!' but the major difference here is that in order to pull those kinds of stunts in EVE you need to be part of a corporation, and it really takes the fun out of it for players who aren't that interested in corporate buggery. This game's world-changing actions have no requirement for large player groups, though players may assist one another.
Say you had a bad day, and you've pumped your space fleet commanding abilities to the max, and you just need someone to take your anger out on. Queue 'Orbital bombardment.' Suddenly, across the server, Heroes and Villains alike are notified that you are trying to ruin someone's day by attacking a planet-side colony from orbit. Of course, if it's just you versus the Do-Gooders, the battle is stacked against you. So initiating a world-changing action creates a force populated either with AI or other, sinister, player characters who will assist you. If you win, the colony is reduced to a burned out crater. If you lose, it still stands, if a little scarred.
Also something I've always wanted to experiment with is a dynamic sword-play system. Instead of having a bunch of characters wailing on one another until someone falls over, have them chose between different responses. Opponent stabs. Retreat, parry, or counterattack? Each action costs in energy, and the lower your energy gets, the slower/weaker your response will be. Retreat and all actions miss. Parry. OK, are you strong enough to stop it, and did you parry in time? Do they disengage and remise(sp?)?
Something like the dynamic swordplay would be best suited to a game similar to battlefront but with pirates and captainable ships. Preferably airships.