Some sort of sandbox game - be it a traditional RPG, a sandbox-strategy a'la Mount and Blade or a sandbox shooter a'la Far Cry 2 - where
a) you are not the (designated) hero - meaning you are NOT The Chosen One, the only person with superpowers or whatever, and your heroic-ness only comes into play if you earn it in-universe,
b) there's at least two, but preferably more, factions engaged in a reasonably fair fight (i.e. one faction may have an advantage, either due to placement, resources, or whatever, but neither side's loss is assured if you don't show up),
c) you can take quests from any side of your liking as long as that faction meets criterion b), with there being at least two options (so it could have some generically Pure Evulz faction that is unplayable, but the other factions are competing with each other as well),
d) the quests are generated based upon the AI faction leader's actual current needs and accomplishing them furthers their goals; if you don't do them, an NPC working for said leader may do it for them instead - that's the main meat of the idea, the quests don't exist in a parallel story-verse where it just seems like you are doing something for a faction but it at best gives the faction 'territory control' they somehow are unable to get on their own.