Music without measures
In Chant and many other kinds of music one often doesn't use regular measures. It would be good to be able to get rid of the time signature and stick in bar lines where they are wanted; or to not use bar lines at all. Also, it would be good to have black (filled in) noteheads without stems for melody lines that are not connected to a beat or specific time (Gregorian Chant, etc.).
Again, it would be good to have a bigger selection of time signatures or a way of writing in something like 5/4, or 7/8 and have the measure length adjust to those signatures. Not all of us are Rock and Pop musicians. Jazz and Classical still exist!