There is another area where WASM could become very interesting; Operating Systems. Or more specific: Kernels.

A nanokernel with the capability to run a WASM binary with a simple GC could have highly portable kernel modules and due to the properties of WASM, run userspace code in ring 0 without much risk to hardware and software.

Additionally stuff like paging and memory management are much simpler to begin with as the memory access pattern of WASM is much simpler (with some dynamic relocation tricks that should be possible you could even identity map and don't do paging at all)

I've also thought about doing microcontrollers as you mention, however, I was thinking about Arduino's (ATmega). They would need a much simpler and more reduced WASM since you only have about 2 KB of work memory.