I'd love to see a Haskell-based alternative to LaTeX. Like the Xmonad window manager, it could become a niche product with its own small but lively community.
It would have great potential because it would provide a very robust building ground for a program.
One of Latex's Building blocks, Metafont, a domain specific language for curve definition (as in fonts) is already implemented in Haskell, and it does not look anything like what people expect Haskell to look like, its part of the diagrams library [1]
What would be next is probably to port the layout algorithm for paragraphs (the rubber and boxes stuff). This could then be used within a framework for page layout.
As a big conceptional change it would probably mean that one separates the Macro stuff from the markup. As writing the plain text within a haskell sourcefile is probably not what one wants (Strings, quoting, etc.).
[1] The diagrams library by itself has resemblances to the immensly powerful Tikz package.
For a while I was working on a Haskell-based macro system that compiles TeX-like source to HTML and LaTeX (and whatever else you like) (https://github.com/jgm/HeX). You'd write your macro definitions in a Haskell program, then run this program on the textual source to produce output. What's nice about this is that you can use Haskell's module system to organize your macros, and Haskell's type checker to check for errors. The macros are just Haskell functions, and the types of the functions determine what kinds of arguments are consumed by the macro. I'd like to come back to this some time.
