I know some have mentioned this, but I'll add to the choir:
Solo Piano generator editor
Fantasy requests that will never get fullfilled:
Being able to create your own melody writer
Being able to created your own soloist
Counterpoint in the harmony generator
More realistic wishes that may be fullfilled soon:
Dobro realtracks soloist
Pedal steel realtracks soloist
More clean guitar realtracks soloists
Jazzy melodic realtracks harmonica soloist - i.e. toots thielman
Melodic violin realtracks soloist - not fiddle - aka more classical
anyway, since everybody is stating what they would like, I figued I'd add to the pile...