keeping the size as it is,

-lighter weight
-xenon flash, LED flash (doubling as video light), better camera, higher megapixel count
-4 row keyboard
-shoulder (trigger) buttons like on a console joypad or Nintendo DS
-better processor/gpu without using up more battery power
-more RAM
-even more storage space, and support for more than 1 SD card at a time (perhaps even the rumoured 1TB ones in the works, probably a while away)
-USB, such as plugging in an external harddrive or a hub
-more than 1 SIM card supported at a time, with options to use 1 SIM for data and another for making calls, or 1 for both and alternating between both, or even better, both are active concurrently. Useful for those with 2 phones, or the possibility of easily switching between an international SIM for travellers.
-digital compass

the unrealistic wants:
-run every video file known smoothly and at maximum (huge) resolutions and via TV out, eg run DVD and Blu Ray files with no need for conversion (when there's no time).
-a smaller (tiny) battery that lasts 30 days without a recharge (or more) with constant heavy use, all connections on even at their weakest levels

I'm not including some expected things like:
-portrait mode support
-emulation support such as MAME (hopefully this is coming).