1) Add back-side illuminated (BSI) sensor to the camera (same resolution) to improve low light performance.
2) Add another 1GB of ram for smoother multi-tasking by heavy users.
3) Remove the sharp plastic bezel that's running around the screen. Make the finish similar to what you see on Nokia Lumia 900 or HTC 1X. This would make the phone feel much thinner than it is and with Gorilla Glass 2 coatings, the danger to the screen is negligible.
For next generation I'd hope they implement the above plus:
1) Make the bezel as small as in the Razr M, and reduce the chin by shoving in a 5" screen without enlarging the physical size.
2) Make the screen S-Amoled+ with current resolution or S-Amoled with 1920X1080 resolution and black levels similar to what's found on original Super Amoleds.
The next gen will most likely come with a next gen quad core CPU, so I'm not listing it because that goes without saying. Same goes for locked bootloaders. They're not going away so I'm not going to even bother listing it.
For those that upgraded now, you're most likely stuck for another 18 months. I'd be willing to bet that in 18 months this is exactly what we're going to see.