I like Nomad, which also uses HCL, but why would you use it for anything else? Serious question, it's not a common choice.
Agree it's not common, don't think I've ever seen it in a non-Hashicorp project, but then declarative languages aren't (relatively!) common.
Alternate tongue-in-cheek answer: if you want a job at Hashicorp? (Or tongue-out-of-cheek: you already work at Hashicorp and are very familiat with it, or looking for a pet project to get very familiar with it.)
Would be interested to hear OP's actual reason though, I'm not criticising the question.
I don't work for Hascicorp and no I'm not looking for jobs right now.
I'm really really curious... a lot!
I was fashinated about how terraform does it's magic, so I studied terraform source code (just the part related to Hcl).
I like the idea to write a custom declarative language, with expressions and custom function.
As side project (just for fun) I'm writing a 2D Cad all with custom Hcl directives and geometric functions (intersections etc.).
Since for my job I need to draw and change..a lot.. solution architectures diagrams... so the Jumble idea was born.
All the best, Luca