The reason for a standard tag language is that the main engine should be able to do some operations on data, like breaking it in pieces like words, paragraphs or staffs on music scores, sometimes without fully understanding what exactly those are.
Possible outcomes: with a proper script language (Lua?), things like tables, multi-column text, and even a lot of crazy ideas could be really easy to write. Plug-ins results would be predictable, since they know nothing about the world except what the main engine has informed them.
As far as I can judge, music typesetting has completely different rules than text typesetting.
Sure.
Ok, you would "just" use another plugin. But then the plugins need a way to interact: captions in graphics (like in MetaPost today), lyrics in music etc.
That's the idea. I do believe it's possible.
I myself wouldn't probably able to handle a MetaPost based system (or something similar) - even if I "speak" a bit of PostScript, I just don't think of graphics as formulae:
Myself, I never touch my computer mouse :) I did read an answer from Donald Knuth on why he thinks Metafont never became a popular tool for font creation. His answer: you can't ask an artist to become enough of a mathematician in order to be able to design his font based on 60 variables.
However, you need different parsers for different types of content
A system like what I want should have a common language for all. I'm not smart enough to say which one.
(I don't speak Lisp. I don't speak TeX-the-language or Lua as well. But the latter seems easy.)
Yes, it is easy and powerfull. And it was created here in Brazil! (Proud smile)
There is your unified system. XML rulez - for better or for worse. It's really no fun to write XML by hand.
But, as you said, TeX and Lilypond have a similar syntax. I belive they could share some kind of common language.
Perhaps you should try to help enhancing OpenOffice's typesetting? # Or Scribus?
I really would like something we could program, and then change design variables at will. Thanks for your comments. I have no knowledge to go beyond what I went in my first message. Best, Maurício