I think it is a great start!
I see you have a constant 0.5 for metallic. If that really was a metallic surface with frost on it, you would want to mask out the metalness (metallicness? metalocalypse?) with the density of the frost. I would also fade between different normalmaps using the frost alpha as well, or maybe just fade the intensity down.
Also, you might want to try having the frost mask accumulate more on the tops of the meshes than the sides. The material function “World_Aligned_Blend” can help you do that. You could use that as the transition phase with the “HeightLerp” function using your frostmask as the height input.
Artistically speaking, you might want to experiment with different styles of frost mask textures on different surfaces. The mask you have there looks kind of like standard cloud noise shapes which could be good for terrain or concrete, but for metal you could experiment with a frost mask that has a bit more of a “crystalline fractal” look, like the frost formed ice crystals on the surface.
Just a bit of criss-cross crystally noise like this picture could go a long way: http://freestocktextures.com/images/small/fst_200afspyi6q.jpg