I don't even need them to change one thing, but some of the organization. Put the *rules* (which, if you think of it, are like 5% of the core books) all up front in one place. Characters work like this. Ability checks/saves/attacks work like this. Spells work like this. Monsters work like this. Magic items work like this. Encounters work like this. Adventures work like this. Long-form campaigns can work like this or like this or like this. Here's a list of things that, taken together, create the sense of a setting.