Hi, there are no design docs at the moment. I am open to suggestions, if you have GitHub account, create an issue there (one per suggestion/feature), if you don't have an account - it's fine to post it here (again, one discussion per suggestion). Though I haven't made a design doc template or guidelines (yet), just write the way you want.
Would be good to know which ideas/features (from other mods, or fresh ideas) people think are cool and will be fun Factorio experience (at least on paper).
I am also not exactly an expert in magic mods, most people would consider me an amateur :)
I already have a (mental) map for the next update: nodes, new metal (probably alchemical brass) and few more effects for runes and pylons. Just need to write the docs.