For starters, I highly doubt a new design will be introduced. My prediction is the phone will still be the iPhone 4 type design, but may possibly sport a larger screen (not by much). It's pretty universally accepted the new iPhone will have the A5 chip, which will greatly improve performance all around. Another, more recent rumor, that's gained a lot of fuel and get's me stoked is a 64GB model. I would be in love if this was true. Next, the rear facing camera will be 8MP and the front camera will be improved as well. Longer battery life will surely be an addition. Rumors also speculate the new iPhone will come with 1GB of RAM, which is a lot for a phone (which are just tiny computers nowadays anyway).
This mostly sums up the big new features the new iPhone will consist of. Also, don't forget it will come loaded with the new iOS 5. Just thinking about this new iPhone gets me excited to own it. I can't wait for October 14th (rumored/projected day of release). I WILL be camping out to buy the iPhone, and I will post pictures and more about the experience.