While writing my guide, I've uncovered two really good free RPG design paradigms. However, you can't really do both and this annoys me. I'll demonstrate:
Paradigm 1. Release small, release often
Taken from software engineering (continuous delivery), this idea is that you don't wait until the game is finished. You wait until there is just enough to play and you release for feedback. Then you build the game in small chunks until it is complete. I like this because you are forced to break the creation into smaller chunks. Smaller chunks are easier to deal with, you get more feedback early and having a conversation with others about the game helps spur you on. I'm trying this with Icar at the moment and although I really like the idea it is a difficult mindset to get into.
Paradigm 2. Design backwards
Again taken from software developement (Domain Driven Design), this idea is that when you design something, you write down what you want the affects on the world to be first, then work out the mechanics and finally write down the attributes/skills. Most game designs begin with statistics and work on from there. I think this often gives you a system that doesn't necessarily fit your original game concept. In those cases, you might have well as started with a generic system.
The problem:
If you design backwards and release often then you're unlikely to release a game that can be played. You will end up publishing the end results of the mechanics rather than the mechanics themselves!