Brainstorming ideas is ridiculously easy . . . it's the deciding which ones to keep and bringing them to life that's the hard part.
I believe it was a Brackey's (?) video that gave this helpful MoSCoW acronym for categorizing what your game . . .
(M)ust have.
(S)hould have.
(C)ould have.
(W)on't have.
Deciding what ideas fit in which categories will help focus your vision before moving forward.
But to this I would also add, that you should think about the real-world difficulty level of what you want your game to do. For example, in your mind you might envision level 2 being super-awesome with 10,000 objects moving in the background . . . only to find out that the game engine you're using can only support a max of 1,000 before the frame rate drops. It's little realities like that which will often force you to come up with creative solutions or add/remove things entirely. It's a constantly evolving process. "No plan survives first contact with the enemy."