You should collect and identify the features of your game. See here a toolbox of features.
A feature is a "A logical unit of behaviour specified by a set of functional and non-functional requirements.” or "an increment in product functionality” or "
distinctively identifiable functional abstractions that must be implemented, tested, de- livered, and maintained"
Divide your game into
Sample of what the features could be:
Core: basic gameplay, leaderboard, score system and score card, city building mode, map screen, levels
Extended: storyline, characters, achievements, sound effects, reward particles, user interface screens, help, tutorial, intro
Wishlist: Bonus levels, mini games, boss fights