I see you are starting from what you want it to look like. That's a good idea! Did the same for Practal [1]. You might be interested in trying to express your language within Practal. Within the next 1 to 2 weeks something you can start playing with should be available. You will then be able to define your own syntax freely (as Practal contains an LR parser, hidden behind an easy-to-use syntax extension mechanism), together with its semantics. By the end of February functionality like execution via rewriting should be available. By the end of March modules are planned, and I think this is when it will start being real fun to use Practal. You could then define your language just as a module/theory in Practal. That's what I am going to do with the language I am working on anyways!
[1] https://practal.com