Have you looked at the Lisps / Scheme / Racket yet? Racket in particular makes it quite nice to go #lang blah at the top of the file and change the parsing or interpretation entirely.

For example all the documentation pages and guides are written in scribble:

docs.racket-lang.org/…/getting-started.html#(part…

#lang scribble/base
 
@title{On the Cookie-Eating Habits of Mice}
 
If you give a mouse a cookie, he's going to ask for a
glass of milk.

And it has an entire document markup language created in it, which can output pdf or html. But you can still use @ syntax to drop in racket code to compute values. Or create templates.

I even implemented a #lang which took assembly directly (and interpreted it, it was for a class).

So if you are really after full control, you should study Lisps and their macro systems.