I've had every generation of Kindle since I beta tested the first generation model for several month before its release. Three and a years ago, I wanted three changes to the Kindle Keyboard: No Keyboard, Touch Display, Backlight. Amazon has nice enough to give me one of those each year and I dutifully purchased each one (it helps to have a large family to give the hand-me-downs to).
After using the Paperwhite for about 6 months, I have no idea what feature they might add for the next model to make an upgrade worthy change.
Unlike the people on this conversation, I don't care about a replaceable battery. Assuming that manufacturers are using a half-way decent battery, that battery is going to last for years and years. Even if its only good for 200 charge cycles, that still 8 years even if you read so much that you recharge every two weeks. Does anyone really think they are going to keep their current device that long?
Frankly, all the things that I can think of for improving the Paperwhite are all evolutionary improvements: Fast page turning, higher resolution, smaller bezel, slightly larger screen, better sidelight. Frankly, the thing I want the most on the Paperwhite is improved software so that I can turn pages by tapping on the left border of the display.
Now, I obviously don't need a new eReader now, but I just ordered an Aura HD because it seems to have all of those improvements I am looking for (except the smaller bezel).