For evolutions I like, I'm going to go back to the original D&D. It's not anything to do with the system. I'm sure there is plenty there but I don't think the system is what made it really special. Between the history books that were coming out, the Secrets of Blackmoor video, and read through of the 1E DMG, and just reading some of the old modules, I don't think the rules were the main evolution, although I'm sure they had some in them. The real evolution between the wargame to RPG was using the rules as a backdrop for the theater of the mind game of "What does you character do?". I think the Braunstein experience of such was the important part that was really being explored in the early games, but being wargamers, they could never allow things like combat to be resolved as such. Thus, the system existed to fall back on in such cases, and they certainly didn't have the experience of vocabulary to explain that let alone offer aid to running it.