For build systems, I always liked the idea of Gradle where the core functionality was simple and declarative, but with the option to use a real programming language for things that weren't simple. For example, integrating installers or form builders (pre-processing) into a build are things I would consider non-trivial if there aren't official plugins, but it was still relatively easy to do with Gradle.
The biggest problem I always had with Gradle was that I didn't like Groovy and I always though there was a missed opportunity to have a statically typed build system with a solid API/contract and all the fancy tooling like auto-complete that you get with statically typed languages.
I see JSON5 mentioned a lot in the comments. In terms of CI / build systems, I feel like something built with JSON5/TypeScript could be really good. I'd be really happy using TypeScript for configuring things like build systems where there shouldn't really be an argument for needing it to be usable by non-programmers.
Personally I feel like I've spent way to much of my life debugging YAML syntax issues.