This is an easy one i guess:
As a guitar teacher I would very much like the global transpose option to be expanded to any key; thus i would be able to, say, play a song in F# (by using a capo) while reading in E.

The option to ad melody and text, in order to create lead sheets, is another league i suppose, and maybe just goes beyond the scope of this straight ahead app; but it would definitely make it my number one app for gigs.