This may not be the next iPhone, but I see these changes eventually happening, when they fully change the iPhone's form. I'm talking like 2-5 years down the road for many of these.
1) I think that the home button will be done away with, at least on the front of the device and they will also move the earpiece for phone calls, allowing for the entire front of the device to be nothing but screen. they could possibly put the earpiece on the back, meaning you would essentially use the back of the phone for calls, but I think that would be a step backwards as far as form goes. At the very least I'd expect the earpiece to be moved further towards the edge to take away more "dead space".
2) better camera + photo editing software built in. I think the main reason apple didn't put a better camera in at first is they didn't want to waste too much time into it when they were building the device from the ground up. A nice camera isn't a game-changing feature, so why bother with it? Now that they are trying to figure the next way to change the game, they will put in a camera and supporting materials that will truly blow people away.
3) full voice control. Not just voice dialing, but voice ipod control, voice texting, voice calendar (ie you have your headphones in say "appointments for the day" and it spits out what you have on tap for today, you say "make appointment for january 10th, 11 AM, dentist" and it puts that in your calendar, voice stock quotes, voice email, voice weather, etc.
4) MobileMe actually works and is included in the cost of the iPhone as a el jobso style mandatory service. I think that MAYBE in two years apple will have got mobileme under control and it will be ready for the big stage. you will have a page that will allow your friends to invite you to events, which then get sent to your iPhone as little invitation thingies on your calendar (denoted by the famous red circle with the number in the corner). Friends (or bosses) can access your calendar.
5) as kind of an expansion on 4, as more and more people get the iPhone, something like "the iPhone network" becomes workable. Meaning apps that allow you to do awesome things from iPhone to iPhone. I can't even speculate here, but I can only imagine what apple can do with 3G and a ton of people using their device as far as creating a nationwide network goes.
6) full blown doc support.
7) full video out. power point presentations, photos, movies, interwebs, etc.
8) better speakers
9) things we never even thought about.