I've been wanting a validator for far too long now :P
Concerning the compiler. I'm considering using @hansihe's compiler as the basis for future compilers, or if we should write our own in every language. One of the cool thing about having one compiler for many languages is that we could then start doing fun stuff like a package and a centralized repository for protodef dependencies.
Take the current varint problem for instance. It'd be nice if we could have one "compiler plugin" that defines varint in the "Protodef Plugin Repo", that has an implementation for (say) nodejs, elixir and rust.
Then when we compile the .json, it references "varint", we get it from said repo, check if it supports the language currently targeted, and compile.
(A fun note: We need to be careful about arbitrary execution)