Elixir. Why? Because it has tooling that is up to the high standards you're used to using in Ruby and Rails. It has a great ecosystem, package manager, and build tooling. The documentation is fantastic and filled with examples. It also has type annotation and type checking and being built on top of the Erlang VM its really good at soft realtime apps which is probably most of what you're doing server-side anyway.
I've used other languages mentioned by others here in projects (Go, Clojure, Haskell, Rust) and although many are nice, for instance Rust has many excellent features and a sophisticated build and package management tool to go along with it, and Go can be somewhat useful if you want a UTF8-friendly concurrent and GC'd C, I still don't think they're good fits for the majority of server-side apps.
I've used other languages mentioned by others here in projects (Go, Clojure, Haskell, Rust) and although many are nice, for instance Rust has many excellent features and a sophisticated build and package management tool to go along with it, and Go can be somewhat useful if you want a UTF8-friendly concurrent and GC'd C, I still don't think they're good fits for the majority of server-side apps.