Flame Body
I love the idea to cripple an opponent with a burn. It's a luck-based ability and we're more scared of EdgeQuake and Earth Power than anything else, and those moves make no contact. Illuminate sounds good, but we're not forced to follow the "is a lava lamp" argument. Using your logic, Ampharos too has to have Illuminate since "his tail shines brightly and can be seen from far away". And Heatran and Volcarona have not visible external flames but can burn. Your arguments are valid, but not fundamental.