A clipboard with some checkboxes, perhaps? Or perhaps a stylized volume knob.
A screwdriver.
The tool, I mean, but if you wanna do the drink, don't let me stop you

Avatar by @DrDubz.
Banner by one of Colin Jackson, Rick Lodge, Steve Noake, or David Severn from Bubsy in: Fractured Furry Tales for the Atari Jaguar.
A clipboard with some checkboxes, perhaps? Or perhaps a stylized volume knob.
A screwdriver.
The tool, I mean, but if you wanna do the drink, don't let me stop you
if the icon is kinda large, some kinda rube goldberg machine diagram
Generic Non-Lego Construction Toy block(s), to represent building the gamemode yourself then if youre so damn smart
A small notepad with a pencil, and whatever is written on it is in a different color than the color of the pencil, and there are some scribbles, some strikethroughs, underlines and circles and arrows in the color of the pencil. It implying that you are using a pencil to change the original text
Maybe like. Building blocks? That or like, a couple simplified sliders I guess
pencil and notebook. you're writing in the rulebook!
My thought is like a triangle circle and square? basically what comes up if you google "shapes icon"