I’ll be running a Development Journal on Fridays. I make unique Pathfinder-compatible content on a regular basis and have strong ideas about the process of designing new content. Each Dev Journal, I’ll pick something I’ve developed and talk about the process that shaped it and the changes it went through. I’ll start with small stuff like a spell or feat, but I will eventually work my way up to full base classes.
Here are the basic principles I work by.
1) Keep a development journal. Keep a log of every idea and every change made. It’s really useful to go back over the log and see what you changed and why in order to make future development better, as well as to mine it for future ideas.
2) Always take power level into account. It’s ok to make something that’s more powerful than what already exists if that’s what you intend to do. It is not acceptable to accidentally make something too powerful, and you should never make something less powerful than existing options. People either won’t use it or will be disappointed if/when they do.
3) Make something new or better. If it exists already, there’s no point in reinventing the wheel. However, if a concept exists that is very cool but the mechanics were poorly implemented, claim it, change it up, and redo it. Don’t ever steal things wholesale, of course. But if, for example, you want to make a sneaky shadow fighter, make one! There’s lots of options out there, but maybe you can do it better or differently.