I’m not sure I can answer this one. Any mechanic I like that one would call revolutionary is probably old enough that it’s been outdone in terms of radical innovation several times over by newer games. Instead, I’ll talk about a mechanic that I really like. It probably wasn’t the first, it certainly wasn’t the like, but it’s the example that I know and like.
Angel included a system for designing an organization for which the characters would work — or own; as you’ll see, the system was designed to be flexible. The GM grants an amount of points, to which the players can contribute some of their own character build points, and the players distribute points into their organization’s spheres of influence and their place within it. The two extreme examples are Angel Investigations and Wolfram & Hart. The first is a group whose players put their points into being in charge. Angel Investigations has minimal clout in Los Angeles and runs lean, without many resources to call on. But the characters are in charge, damnit: they answer to no one but each other and set the direction for the company.
Alternately, Wolfram & Hart’s players clearly dumped all their points into contacts, resources and clout. The firm exerts influence on multiple planes of existence, its facilities seem limitless and people generally quake in their boots when J.Q. Cheatem from Wolfram & Hart shows up with a sheaf of papers in hand. On the other hand, to get their law firm to that level of status, the players have put themselves at the very bottom of a very tall pyramid which is capped off by three or more demigods of uncertain though certainly worrying levels of power.
So there’s a wide spectrum to work within when designing your organization with Angel‘s mechanic. I really dug that, and regret not having had the chance to put it into play yet. Giving the players that level of control over the campaign framing device should really get them engaged with not only their character’s development, but the course that organization takes over the span of the game.