RPG

D4, D6, D8, D10, D12 and D20. It's all you need.

Mechanics

Rule Systems

The rule system handle the progression of the player character and the rules of the game world.

Quests

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 Quest for the Custom Quest System - 2017

Journal

The system in the game that track the players quests and show what he should or could do next.

Dialogue Systems

How the player speaks to non player characters in the game.

Maps

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.

Reputation systems

Crafting

Inventory

Designing an RPG Inventory System That Fits: Preliminary Steps

Game Design Essentials: 20 RPGs - 2009

NPC schedules

Games

Why RPGs are so hard to classify and evolve? - 2016

The CRPG Book Project - 2016

Wikipedia: List of role-playing video games

The CRPG Addict

Putting the Role in Role-playing - 2017

Playing Roles: The Philosophy of Design - 2015