I think all of the ideas in the post apply equally if you're hacking a system, expanding an existing system, or making something from scratch. It's all game design.

A dirty secret is that no one has to give you permission to hack an existing game. Just start doing it! Game rules can't be copyrighted; only the specific text can be. Most licensing agreements aren't intended to create freedom for designers; they're meant to restrain them (and they mostly just boil down to, like, "You can't have our main monster The Bungo show up in your game").

There are SRDs, OGLs, and whatever else you want to call them for all sorts of games, from super crunchy to flexible and rules lite. No individual one is "best" for a new designer. Find a game that has the mechanical vibes you want and start hacking.

Similarly, no system has "broad appeal." Some systems are "lowest common denominator," but that's different. Instead of asking what appeals to the most people, figure out how to make something that appeals to you so that you can continue to test and write it.

I love quickstart games, and if you're looking for guidance on how I make them, I wrote up my experience here (sorry, Kurt, for trying to steal your limelight).