Almost every phone I had before (including some 6-7 years ago) had programmable convenience keys. You, those ones you press and they take you straight to an application. It would be really convinient, say when you are listening to music and don't have time to find the Camera app to snap an amazing moment. Or to take you straight to your texts. Customizing it would be the most important feature.
Just my two cents.
Just my two cents.