I mean any kind of mix of 2 or more fractal versions (with merge shader, or stack some nodes, use blue node add color, mix color, subtract color, multiply color, etc), and also the settings in the noise itself, like variation, buoyancy, clumping, even extreme or negative values sometimes yield interesting results. And plug that into density input, or add other shaders for the depth modulator (which causes clouds to vaporize/vanish due to the added noise whites, kind of).