Since there are a couple of demon skins in the game im thinking of a demon Junkrat skin.

Two volcanic like exhausts coming out of his shoulder blades kinda like torbs exhausts. Junks hair could be pure fire and magma with two large demon horns, one being partially blown up. A demonic style outfit for him that fits his character and the skin, His metal arm could be full of bombs like her could just load it into his gun, Junks peg leg could be a boney peg leg thats always on fire (Like Ghostrider for example). Junks trap would look like a lot more rusty and have an obsidian or magma shell, Junks grenade launcher would easily be Semi melted in appearance with small bits of lava dripping from it and finally Junks mine could be a mine shaped demon that junk uses like his normal mines