Object-Oriented Game Systems
Random snippet of thought: Rodger over at Metagame posted a while back about encapsulating mechanics within characters. His example contrasts how a Roman and a Celt might have different internal mechanics which express the same final game result.
I really, really like this idea. I wonder how workable it is generally — to create basically a game API, and allow the players to construct their own tools to talk to that API. The idea of balancing something like that terrifies me, but it’s so neat.