If you're looking for a configuration language that is powerful and useful AND can interface with existing YAML based systems, may I recommend Dhall?

https://github.com/dhall-lang/dhall-lang

Dhall gives you the power of a limited programming language, the power to version configs and perform generation and logic in your configs, gives you the ability to spread your configs and custom functions for config generation across multiple machines safely, and does so with good guarantees about the type system and does a good job of stopping potentially infinite loops.

Dhall is really, really good and very overlooked by the industry just because its syntax is functional.

What kind of things have you used Dhall for before? I think it's pretty interesting but we don't have enough problems with YAML to really switch.