1. Organize the books (in particular the DMG) with an eye towards making it easier for new player, DMs, and groups as a whole to pick up the game, avoid pitfalls (particularly those that might make someone quit before they really experience the game as it will play out once they have learned the ropes), and get a foundational basis into modes of play the system supports.
2. Include sidebars discussing the reasons for specific rules, what purposes they serve, and how they can be modified (including potentially otherwise unforeseen consequences of doing so) to achieve specific ends.
3. Include (and focus on) generalized task resolution -- physical, social, mental, combat, non-combat, long- and short-term. Give examples, but make clear that these should be extrapolated outside the set boundaries. Special focus on the things most often resolved in D&D with a spell cast, or actively boring when performed in current 5e (especially by those most likely to be engaged in such actions, such as someone playing a ranger with wilderness activity).