So basically you don't have an idea at all.
You saw a game you liked and want to do something similar. You have no idea, and you've come here hoping we'd give you an idea.
I do have an idea. My Idea is to use the basic game play of doodle god and make it into an entirely different game. In doodle god you drag pictures onto other pictures and that is it. This game will be a 3d first person sandbox free world environment type game. So for example, you find some iron in a cave (which you created) , you combine it with your cart that you made previously. And Bam. You have created a car which you can actually drive to run down those zombies you accidentally created.
This game will eventually seem so unlike doodle god that you will not even see the resemblance. (maybe)
FYI: If you made a FPS wouldn't that be copying call of duty? So before you say im just copying someone's idea. Im not. Im copying the style of gameplay and that is all.
So Yes, I have an idea, and all im coming here for is help making it because im not good at 3d game maker. Im not asking anyone for an idea.