What I'd really like to have is a sort of "framework" function (using preg_replace or whatever) allowing to develop and use tags with parameters. Then a page could contain different instances of a tag that would produce different outputs based on tag parameters.
Syntax could look like {MYTAG:var1=int:var2=string: ... :n=zzz} - similar to typoscript's.
Say I'd like to have a "scattered" navigation menu:
{MENU:level=0:position=hor:style=1} would output
Horizontal 0 (root) level menu with predefined style #1
{MENU:level=1:position=hor:style=2} would output
Horizontal 1st level menu but different style (#2)
{MENU:level=2:position=vert:style=3} would output
Vertical 2nd level menu, again different style (#3)
I started doing it for my own purposes, wrote menus-producing functions but stopped at content.func.inc.php as I simply dont't have enough technical knowledge of how to parse and replace my tags with function using tags' parameters...

I think it would be a very useful feature.
Marius