I have a feature I would love to see - SOLAR CELLS!!!
Stick some solar cells along the periphery of the screen to supply the battery with supplemental power while the phone is in operation. this should extend the life of the battery considerably. The technology exists for embedding miniature cells in devices. For example, Citizen uses it in most of their watch line today (that's what eco-drive is).
With all the demands put on smart phone batteries, I'm surprised that such a supplemental power system has not been tried. As I see it, this would be especially beneficial on Tablets.