You may want to take a look at Pixie.

"Pixie is a lightweight lisp suitable for both general use as well as shell scripting."

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

Thanks for posting your config. I've learned a lot from it. In return, here is my Fish config. https://github.com/jramnani/dotfiles/tree/master/fish

As you'll see, I like the way Fish automatically loads functions from, "$HOME/.config/fish/functions/". It keeps my config.fish file pretty lean.