What can i do as a final peice for my Art GCSE?

You could try painting of extreme weather/ its effects, or drawings or whatever, or both. Or you could use a canvas and create almost a 3D painting, building up bits you want highlighted with paper mache, and painting over/ drawing using biro and pencil. Or you could partition it to show different types of extreme weather, like the four seasons type thing. That's just an idea, hope it helps.