After musing on this idea back in April, I've now begun to work properly on a complete D&D rule book for a future campaign.
I've taken the Swords & Wizardry core rules document as a starting point, and started hacking away... What fun!
What I'm thinking is that making a pretty decent set of rules booklets based on the S&W rules is so simple that each campaign can easily have its own. I usually find that each idea I have for a campaign comes with a set of house rules, alternative or disallowed classes & races, new spells, etc etc, so being able to relatively quickly write them all up into a proper rules document seems like the perfect way to go.
So this is not so much "D&D Mine" as "D&Ds Mine".
The current D&D Mine is a rules set for an early modern inspired, low fantasy setting which I tested out a bit with my recent running of the Grinding Gear.
I'll post progress reports as significant things happen :)