Looks interesting. I had spent a while playing around with LilyPond (http://lilypond.org/), and I think something like Handel would solve issues with some of the more repetitive parts of using LilyPond.
One thing that would be helpful that I didn't see would be able to write transformations of chunks. For a few examples: (1) taking a chunk and replaying it in a different scale (or with a different starting note), (2) having the end of a chunk resolve to the tonic up an octave, (3) inverting or chunking chords following common patterns.
It would also be great if instead of just outputting midi, it could also output either to engraved music, or to some other format (e.g., LilyPond) that someone could then use to create a PDF.
Lilypond was created for engraving music (aka "sheet music"). Handel has essentially nothing to do with that at all. Handel is a compositional "tool", Lilypond is a music typesetter. There are all kinds of tools for composing/writing music; Lilypond is essentially a backend for such applications to allow them to generate beautiful sheet music.
One thing that would be helpful that I didn't see would be able to write transformations of chunks. For a few examples: (1) taking a chunk and replaying it in a different scale (or with a different starting note), (2) having the end of a chunk resolve to the tonic up an octave, (3) inverting or chunking chords following common patterns.
It would also be great if instead of just outputting midi, it could also output either to engraved music, or to some other format (e.g., LilyPond) that someone could then use to create a PDF.