Another thing, I guess, is making it easy for musicians without
programming knowledge to smoothly embed their own articulation signs,
note heads, clefs, and other font symbols into LilyPond at runtime:
Just define a new articulation sign or note head shape or clef at the
top of your .ly file with a single short line of scheme code that
references some, say, .eps file. I think this is still not that
easily possible, right? (Please correct me, if I am wrong.)
Personally, I am even not sure of how to properly notate contemporary
music. Yes, I have seen e.g. excerpts of Stockhausen's score of his
Studie II, and Wehinger's aural score of Ligeti's Artikulation, as well
as a score of Kagel. (LilyPond's short, long and very long fermata
signs were actually inspired by this score of Kagel.) Still, I am not
satisfied with such notation: At least to my perception, it typically
does not represent well essential nuances of e.g. electronic sounds.