This is a really, really good idea, and I haven't seen one of them out there. I'm actually starting to develop one, and will put it out there for comment once I have something that is worth sharing.
Phew, even after having done this for awhile there is a lot that goes into making a how to guide for writing subclasses. I've got a first draft of the Barbarian, Bard, Cleric, Druid, and Fighter written. So, hopefully some time next week I will have a first draft of the entire thing for other to take a look and and disagree with me about my assessments.