[cutler]

Kotlin is a good transition language from Ruby. Concise with a nice balance between FP and OOP and all the scaling benefits of the JVM.

[vasergen]

What about Elixir, seems to be a good option as well

[cutler]

Not if you want to retain the OO features of Ruby. Beyond the surface syntax similarities Elixir is at the opposite end of the spectrum compared with Ruby.