R2 and L2 buttons would also be nice and a better battery life would also be nice additions. A man can dream at least.
It seems having the L2 & R2 buttons is one of the biggest feature request for handhelds but I can't see it being added unless the design of the handheld is going to be bulkier. I'd love to see how one could incorporate that into a design without making the handheld much thicker.
There's this add-on I saw which looks comfortable on hand, but not in your pocket: