So, one thing that I've been thinking about lately is temporary mechanical bits that get added to your character and modify either the way he acts in the narrative or the way he interacts with the mechanics. This could be something as simple and video gamey as a status condition - like "blind" or "on fire" or whatever - or something more story game-ish - like temporary Aspects in Spirit of the Century. In my mind it's a nifty technique that we (in the sense of the collective aggregate of gamers, as well as the more specific sense of hippie gamers) haven't nearly wrung dry. Seriously, lots of mileage there yet.
To my mind, the best use of these is something that a) expands, rather than limits, the players options; and b) leaves the choice ultimately in the player's hands. I know that the second one seems to follow logically from the first (and it does, really) but I'm saying specifically that it should be the player's option to use these or not, at least under most circumstances. This puts me in mind of the way the Secret Arts work in Weapons of the Gods. They place a chi condition on your character, which either gives you a bonus for taking them into account in your decisions and descriptions, or a penalty for ignoring them. Aspects also work this way, although on the surface they appear to limit choices; after all, that's what Compelling an Aspect is supposed to be about, right? But Compelling limits the character's choices, not the player's; in fact, they expand the player's choices, because he can still do whatever he would have done in the first place (albeit with a cost, by refusing the compel), and has the additional option of gaining a resource (Fate points) if he accepts the limitation. So, both these implementations get an "A" in my book.
But there's a key difference between the two. In Spirit of the Century, the Aspects are very off-the-cuff and in-the-moment. They're not really intended to affect behavior over the medium- or long-term. Weapons of the Gods is kind of the opposite; these Chi Conditions work as kind of a buff or debuff (to use MMO terms). They're usually set up in advance, and are more a way of subtly manipulating things in your favor than a neat trick to pull during conflicts.
So, finally approaching something like a conclusion: I'm thinking there's a lot of potential in something that kind of combines the two.
And yes, this is part of the seed of the idea for a game in my head.