How they basically could not improve on tagged union enum types as you express them in C is to me one of the misses. Modeling data through effortless sum types is a requirement for me when picking up a modern language.
Also, gradual typing + ADTs: Crystal, Elixir and Erlang typespecs, Haskell, Python
Finally, dependent typing + ADTs: Ada 2012, Agda, Futhark, Idris, Lean
Without naming names, there is a language out there with some overlap with Zig's domain that does have really good tagged unions. But that project has a different set of goal than Zig's and that's okay. They're both good projects.