IMO one big advance would be writing the compiler in something other than C. C is a bad language for writing compilers in (including C compilers)!

Even (cough) C++ is better than C for this use case, e.g. comparing LLVM vs. GCC. But C++ has problems too though -- the TableGen language in LLVM is an interesting example of why a big C++ project like LLVM/Clang doesn't want all its logic/specification in C++. They really use C++ to the hilt and they still want/need a DSL.

Other interesting options would be Swift, Rust, OCaml, or Go. Swift, Rust, and OCaml all have algebraic data types.

I guess choosing something other than C or C++ is a little unusual because there is the tendency to self-host. The Rust, OCaml, and Go compilers are written in Rust, OCaml, and Go, respectively. I'm pretty sure Swift is a lot of C++, but that will probably change as times goes on.