I guess Savage Worlds? It's been a long long time since I've seen a change made to the core rules that felt energizing and fresh. The system takes small tweaks and changes for settings very well, but to me a "hack" is running with the basic mechanics until you get somewhere else (but I allow that maybe I don't understand the term fully). The only examples I can think of are the superhero rules for Necessary Evil and the dueling rules for Deadlands.