RPGD4, D6, D8, D10, D12 and D20. It's all you need.
The rule system handle the progression of the player character and the rules of the game world.
What the player need to to next and his progression in the game world is tracked by quests. Each quest is a single task that needs to be done.
The system in the game that track the players quests and show what he should or could do next.
How the player speaks to non player characters in the game.
Exploring the world is common in role playing games so a map system is important to show the player where he is in the world.
The CRPG Book Project - 2016