Elixir - Tried Erlang a long time ago, totally sold on Lisp and message passing since then.
Julia - Interested in multimethods and sort-of typing as core language features.
Both of them: I miss macros.
Rust - Some of my hobby stuff is in C/C++ but I don't particularly enjoy the compilation model or lack of safety.
Out of the three, Elixir is probably the most immediately relevant to me (web developer), so that's probably what I'll end up exploring next.