Off the top of my head, here are some things I'd like see:
- HDMI out for HD video
- Integrated GPS
- Better still camera
- Video camera
- Voice dialing
- Bluetooth stereo audio out
- Obviously, more storage
- The ability to run regular Mac OS X apps when connected to a full-sized display
Ultimately, modulo Moore's Law, there's no reason I shouldn't be able to drop my iPhone into a dock with power in and HDMI audio/video out, pair it with my Bluetooth mouse and keyboard and IR remote, and use it like a full-sized Mac—including watching movies and playing games in 1080p HD video—then undock it and carry it with me, using many of the same apps but in their iPhone-native modes, with additional mobile features like GPS.
It's astonishing to me that Apple is expected to sell more iPhones than Macs this year, and maybe 3-4 times as many next year. (Or, if you take into account both the iPhone and iPod Touch product families as does Ross Garber, maybe 8-10 times as many.) Combined with the fact that only 25% of iPhone users are Mac users, that means Apple is making incredible inroads into the Windows PC customer base.
If Apple gets serious about adding features like those I've listed here, who knows? Your next PC might not be a PC—or a Mac. It might be an iPhone.