CannonBall or BasketRock?

I was not thinking of concept from the begining. Firstly, I wanted to make kind of a little dining room with "cannonball" that shoots with tennis ball and there would be many different baskets on various heights. On the halfway, I just wanted to finish the project and this ... is it.. I guess

Please, tell me how do you come up with ideas? 

I mean we are not all concept artists but modellers, so our image in the head is so vague and sometimes out of our skill level. How do you cope with this?