Principia reached an important milestone in my creative process today (though it's one I wish I'd reached a month ago). The rules document is now concept complete. This means that everything I think needs to be in there is in there. A few sections are merely skeletal, but to my knowledge there's nothing missing.
There will still likely be changes, additions, and subtractions, but it's all downhill from here.
Also, it looks like Brandon Amancio is going to step in as co-author. I've been having a lot of trouble getting the secrets and setting stuff to come out right. Brandon is a genius at just this kind of stuff so he's the perfect person to bring on board.