Like we say “Appetite comes from eating” good idea come when your making a game!
A prototype can be lead by a small mecanic like “a platformer where you play only with the mouse position”
Do not make any design for the game focus on the mecanic , test new things!
Challenge you to make ten game prototype like BlackTornProd as do.
And maybe in these 10 prototype you will find a good game idea otherwith you will have learn new coding trick or new way to do something.
You will train your self to be a better GameDev !