One milestone I will be looking forward to in the WebAssembly space is when a compiler that can output WebAssembly has been compiled to WebAssembly and is capable of compiling itself. The first compiler to do this will be the first native WebAssembly compiler. Right now, all compilers that output WebAssembly do so as a cross compiler, where the compiler is one instruction set (x86_64, aarm64, etc.) and outputs binaries in a different instruction set (WebAssembly aka. WASM).
I think we are both a ways off from this happening and much closer than people think. Really, all that needs to happen is for someone to do the work of compiling something like clang to WebAssembly with WebAssembly target support. The requisite operating system support has already been specified with Web Assembly System Interface (WASI).