Some semblance of text editing. Right now it's a pain to edit text written in imessage or and of the default apps. How about putting in some arrow keys on the virtual keyboard so you can move your cursor around and edit without needing to resort to simply re-typing everything out again as you can't simply back up and change or edit some letters.
You can edit text already on any iPhone by pressing on the screen until the magnifying circle appears, and there are cursor keys in landscape mode on the 6+.