This, lisp is perfect for representing arbitrary data, nesting is just another sexpr, easy to produce, easy to parse and easy to debug / reason about

I can appreciate this answer, but I don't think it's really what I'm asking.

I think I'm more looking for some kind of standardized struct definition that translates easily to llvm IR and is flexible enough for a wide variety of languages to target.

Something like this: https://gist.github.com/thomaswp/8c8ef19bd5203ce8b6cd4d6df5e... (Which doesn't meet my criteria because AFAICT isn't used by anything, but is reasonably close to what I want) or this: https://docs.rs/sap-ast/latest/src/ast/lib.rs.html#1-83 (which seems specific to SAP, I would like something more general)