Most of my personal experience with successful long running (1+ year) games has been with mostly just sticking to the core rulebook in games like Vampire: The Requiem, Exalted, Classic Deadlands, Changeling: The Lost, Demon: The Fallen, Legend of the Five Rings 5e, Infinity, Dune 2d20, etc. Granted our games tend to focus more on social conflicts, characters' personal lives and intrigues. Violent encounters are rare and fair fights damn near nonexistent. Our year-long weekly Infinity game saw maybe 8 combats and very minimal exploration but featured well over 30 named NPCs (at least half of which had direct associations to at least one of our 3 PCs).