|Eight of Wands|
I allow the players to contribute, both knowingly and unknowingly, in and out of character, to the shared reality of the game. I incorporate elements of the PCs' machinations and the players' wild speculations into the story--sometimes directly, sometimes retroactively, and sometimes in barely recognizable form. In effect, this inflicts a limited version of the Bard's Tongue flaw on every single character.
As I do this more and more, I realize that I will eventually hit a wall, which is the fundamental structure of a game like Mage--and most other tabletop RPGs--that separates GMs from players. Every now and then I halfheartedly investigate game systems that either distribute GM responsibilities (e.g. Prime Time Adventures) or have no GM at all (e.g. Narrative Cage Match), but so far I have not found exactly what I want.
What exactly do I want? I want a fully collaborative storytelling game system where each participant is both a game master and a player. The Narrative Cage Match system for the game Patheon is fairly close in concept, being a kind of themed Exquisite Corpse with dice and counters. Its mechanics, however, are centered on competition and scoring, and gameplay seems heavily reliant on the scenarios (essentially modules) presented in the rulebook. I am looking for something a bit more free-form than that, but not so free-from that it devolves into the Oldest Game.