1The fixed part
Movement, collision, speed, HP, spawn rhythm, score, leaderboard, notifications. These decide whether the game feels good, so they stay fixed.
2The swappable part
World, character names, colors, images, copy, audio mood. These decide whether the game matches the idea, so they can change.
3The one test
If this changes, can the game become harder or easier?
Yes: lock it in the engine. No: put it in the cartridge.
4What Remix keeps
Story, characters, and visuals can change while movement, collision, difficulty, and scoring stay stable.