I read the links; your language looks interesting, but could you expand a bit on what makes it a new approach? User-programmable syntax is an old idea; for instance, it's been a core feature of Scheme for 30+ years.
My language is not only about user defined syntax; it's also about user defined code generation. For example, you can define your own syntax that compiles directly into a DB stored procedure or into GPU shader, or into whatever technology that might be invented in the future, like for example quantum computing. In fact, it's less about user defined syntax than it is about user defined code generation as I am trying to limit new syntax into certain patterns rather than leave it in the wild.
Does scheme give you access to the compiler's internal data structures? Can you for example write an extension that scans through the compiled code looking for, say, for loops and replacing them with something else?
Macro expansion happens just before compilation; you have the chance to alter the source code as it's being read by the reader just before compilation.
But I see what you're getting at; you mean global modifications to the compiler to interpret the whole language differently. A macro allows you to make syntax, but it doesn't change the meaning of other syntax elsewhere in the program; only allows the programmer to add syntax to the language.
And yet Scheme hasn't turned into something like that. There's a limit to how radical a syntax you can achieve using (define-syntax), and there's no provision for modularity. Worse still, syntax definitions are "imperative" - it's really hard to see what they do by looking at them.
I'd like to see a system where you have tiny, composable pieces of syntax written declaratively in a domain specific language that's as terse and general as possible - stuff like "a[n] -> (list-ref a n)" and "a..b -> (iota a b)". You want to make it as easy as possible to port language concepts to the system. Most of the wacky semantics you can get in other languages can be had in Scheme through some sort of library already, they're just usually kind of a pain in the ass to use without the syntax. Then making the language of your dreams is just a matter of enabling all the syntax you like. You'd need some kind of mechanism for handling grammar conflicts.
Of course, it's really hard. "Syntax" masks a lot of complexity. A feature I want, for instance, is to have the whole language behave like a CAS, where instead of bombing out on an undefined symbol it just computes as much as it can symbolically and hands it back to you e.g. "x+2+3" returns "x+5" if x isn't defined. That sort of thing has been done in Scheme of course, but it's hard to imagine a syntax specification language that would let you just turn that on as a single "feature" without massively affecting basic things like expression parsing grammar.