So what if you have been working on this kind of thing for a decade and already published some of it but not all? This is exactly what I have been doing with my Majestic Fantasy RPG.
Would an extensive design note booklet (like a zine) be in order? For example, I have a couple of dozen monsters but the "additions" that are common to all to make them work can be summed up in a paragraph. One of them would be that monsters get an initiative bonus equal to 1/2 of their Hit Dice rounded down.