I'm on a design roll lately. I have another supplement that will be done quite soon, Traces of Magic. It's an attempt to take OSR magic and make it and it's traces more diegetic, specifically that magic leaves residue (of various sorts) that can be sensed by characters with the right training.
Why? Several reasons. First, it adds a wrinkle to dungeon play, it's one more thing you can add to room descriptions, that magic was done here, that the characters have a system to explore further. Second, and maybe more why I wrote this, it makes magic and the doing of magic something that can act as a clue for investigative scenarios after the fact. In most games the magic gets cast and disappears like it never was. I want our PC sleuths to be able to not only 'taste' the magic to get an idea of power and recency, but I also want the caster of that magic to have a unique magical fingerprint. Obviously, I'm drawing heavily on vestigium and signare from the Rivers of London books, but that's the functionality I want.
Anyway, it's an idea that been in the back of my mind for maybe years. IS this an idea that you guys think you'd be able to make some good use of in your games?