There was an interesting talk on Game Access (What the hell is Game Access?) conference about character design for games by Konrad Czernik from Techland. They made games such as Dying light and Call of Juarez. He gave a few great tips on the topic including that your team should have an art guide.
In short – guidelines on how to make art for your game. This includes everything like color palette, resolution, aspect ratio, perspective, visual style, character sizes, your dev pipeline, …
Good examples are Dota 2 Workshop or Liberated Pixel Cup.
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