It would obviously not be called the iPhone 4G, as it would have to support LTE (only 4G network) which it won't until at least 2012. I'm hoping for HSPA+ (21.6 Mbps) support ... in Canada all of our mobile carriers are on HSPA+ and support tethering, so it would be nice to not be restricted by the iPhone's slow HSPA-only capability (7.2 Mbps). Unfortunately, AT&T is stuck in the stone age down south with their HSPA network at 3.6 Mbps, so I'd be suprised if Apple added HSPA+ support.
That and better battery life, possibly through smaller fabrication process of the CPU, would be the two top things I'd like to see ... oh and out-of-the-box support for some sort of multitasking (maybe running 4 apps simultaneously)