I feel like ruby (or a DSL written in ruby) would be the ideal language for writing CI pipeline "configurations".

You'd end up with config files that are actually readable (even to non-rubyists), with a shallow learning curve, but still easily extensible.

Here's a 5 minute mockup: https://gist.github.com/gkampjes/559ebe708343bb7c2f590c0065d...

Looks nicer than any .yaml file (or Groovy script) I've ever seen.