I've been working on a simple homebrew for years now. Its fairly simple and focuses on fundamentals. I began with definition of an RPG, a glossary of terms related to the hobby, and then instead of going into the rules I realized I had to go into "Running a game". This meant not just the GM, which is a lot of the work, but also the player-side as well. It can go long so I would definitely just word it as "basics", focus on core principles, and tackle more in a later chapter.
Focusing in a particular set of GMing is the part that is puzzling since I can narrow it down with better ways to express it, but I dont want to come off as this is the only way to GM. That problem that I have a set of mnemonics and ways to juggle and run games, and break down the complexity but my method is not the only one or the best one for you - but the method that is easy for me since I work with certain soft skills that I value in daily life. :P
Anyway part of my current line up of articles is Non-Binary Consequences. I'm working on some tools so that the GM can easily improvise on such (using 2d6 tables of word formulas). How to manage the valleys or low points in pacing (the perfect time for building player buy-in). As well as some dice tools (again 2d6 tables with word formulas) of how GMs can draw out tension. Well I hope I get it all done, the articles are half written with me leaving a ton of the problems to be worked by my subconscious.
So you can imagine how much it has evolved from 2015, my anticipation is that I will have more work and more responsibilities so gaming has to be more portable, Intuitive and more "Ice-breaky". That I can quickly get a regular person, non-ttrpgamer, to try it and play to kill time. So it has to be physically and mnemonically portable, and probably great training for sales lolz. Truly general universal role-playing - in what ever system the audience is inclined to.
EDIT: I'm incorporating this Extra Credit, currently working now on how to implement the ideas and giving examples of such examples. After finishing it, I have to rethink a lot of what I just wrote above and design it around an adventure. I will have a rulebook and the Tuturial Adventure. Tutorial Adventure first, then collect everything as a rule book to follow.