This is a moderately comprehensive guide on how to create an artstyle for your game. Even though this mostly covers examples of 2D game art, both pixel and Handrawn, the principles will apply to 3D games as well.

I haven't named any artstyle genres such as pixel art or cellshading, because at the end of the day I don't think these terms matter too much, since you can make endless of stylistic decisions, there will also be an endless amount of 'artstyles' some stylistic choices will obviously have a larger impact on the 'look' such as voxels or lowpoly, but I don't think you should limit your idea of styles to such a restricted view.