Apple can put a better camera in it, add more capacity, a better processor, a front-facing camera for videoconferencing, and a dozen other things -- but quite honestly, I'd be perfectly happy if none of that made it into the next-gen iPhone so long as one feature does find its way in: support for 900 MHz UMTS/HSDPA.
The iPhone 3G and 3GS, as well as the forthcoming iPad, all contain 3G antenna/chipsets that support UMTS/HSDPA at 850, 1900, and 2100 MHz. UMTS/HSDPA is an alphabet-soup way of referring to the GSM flavors of 3G. In other words, high-speed broadband works at those frequencies, and only those frequencies. If high-speed broadband isn't available at those frequencies, the iPhone will fall back to GSM/EDGE at 850, 900, 1800, or 1900 MHz.
For those of you living in the US and on AT&T, the iPhone's 3G frequency range is especially tailored for your use. Most of AT&T's 3G network runs on 850 MHz, while T-Mobile runs at 1700 MHz -- this is why the iPhone only works at EDGE speeds with T-Mobile, because the iPhone's 3G chipset isn't tailored for the 1700 MHz band.
3G networks in many other areas of the world, including portions of Europe, Asia, Australia, and New Zealand function at 900 MHz. This creates several problems for users in those regions; read on to find out why.