The Concept
I want to create a rhythm platform game where the user has to hit a button for each beat of the song. If the user fails to do so, they will lose and will have to repeat the level.
The game will be something like this, where players must hit a button and it’ll perform an action, maybe the action is to jump, hit, duck, etc. 
The player needs to be listening to the song in order to know what to do. But the game will also have visual cues so the player isn’t entirely reliant on just guessing the beat.
Design
I want the game to have a bit of a retro feel, almost like a musical Mario game. The game will consist of pixel art and will feel almost nostalgic. I want to highlight the style of gaming that popularized platform type games.
The Most Frightening Part
I’m mostly worried about making the key the player presses in line with each beat of the song. It being a platformer minimizes the risk, but how big is the margin of error for the user? Additionally, how can I make it so that the player needs to do an input at specific beats of the song.
Minimizing the Risk
I did my best to work with sound in p5js and tried to make it so that the background color changed during the beat of the song.
song.addCue(2, changeBackground, 'palevioletred'); song.addCue(3, changeBackground, 'mediumvioletred'); song.addCue(4.2, changeBackground, 'pink'); song.addCue(6, changeBackground, 'orchid'); song.addCue(8, changeBackground, 'skyblue');
I used addCue to change the background during certain parts of the song. I probably could have put it into the loop but I first wanted to just highlight parts where I heard a beat and put the changeBackground function there. I also think it’ll be funner if the beats aren’t entirely consistent, as that would mean the levels will be harder.