Cannot talk about functional alternatives without mentioning Haskell. OCaml (when abstaining from the "O", as many OCaml'ers do; similarly Scala'ers often abstain from the "O" in Scala) is also an interesting option.
Finally there's Rust, which is besides being a bit more functional also more low-level than Go.
While being fairly young, Frege[1] also deserves a mention. Very similar to Haskell, but on the JVM.
I had forgotten about Rust. Are there major projects being used for this yet? I've heard it's picking up quite a bit.
That's the same as saying:
Well, I was tempted to mention Go. Problem is, I haven't found a practical use for it.
That is to say, just pick a problem and try to use Haskell to solve it. 90% chance it will fit your niche fine.
There's a lot of other usage too, it all depends on how you define "major".
Personally, I prefer to write code in a functional manner. While I've always thought Go looked like an amazing platform for programming in general, I haven't been keen on moving to another imperative language.
It seems the landscape for functional alternatives are mainly Scala and Clojure which are both based on the JVM and require a bit of time to learn the tooling. I am not a Java or JVM export, so I haven't been too inspired by this either.