One thing that would be really cool, and I don't believe you can do it at the moment so please correct me if I'm wrong. But it would be fantastic if you could highlight keys in the Piano Roll. So lets say I select E Major...
Also laying down chords, it would be fantastic to be able lay down chords without any knowledge of chord notation.
On a more techy feature, how about some kind of intellisense feature that uses basic music theory to check your arrangement and notify you of possibly errors, such as notes being out of key and incorrect chords. That would be wizzy but the feature above would be really fantastic.