After playing with an iPhone 4 for a week now, I can't think of anything else I really want to see in a future hardware revision. The iPhone4 is practically perfect.
I've been really upset at US Carries whom, for years, have removed front-facing cameras from phones before they hit the states. HTC made a ton of Windows Mobile devices with front cameras but AT&T, Sprint and Verizon always had them removed. Now I have a front facing camera.
I can't really tell the difference between 720p and 1080p, so I think the 720p shooter in the iPhone4 is great. It takes great pictures and video, but doesn't adjust well to white balance changes I've noticed. It also looks kind of overexposed in video most of time, so I think there could be some change here, but I don't think I'm really interested in things like a 8MP camera with a huge zoom lens.
I imagine next year's iPhone will be very much like the jump between the iPhone and the iPhone 3G. We might LTE data and increased storage capacity, but not much else and I'm fine with that, as I think the majority of people are.
All the changes need to happen in the software. The iOS UI is getting a bit stale. I'm tired of swiping through page after page of grid icons trying to get what I'm looking for. I'd really like to see an application pull up menu like in Android or WebOS so that I can have a bunch of stuff installed but only what I use on the Desktop. I'd also like to see the equivalent of widgets for iOS.
It just feels like something has to change UI-wise in iOS. I think it was a great shot out of the gate in 2007, but things feel different now in the smartyphone world and I don't think Apple is doing enough to adapt to keep it up to date and feeling fresh.
I think I'm fine with the notification system, but I would like to see some improvements like QuickReply. Being able to reply from the popup would make life a lot easier but it also needs to be taken one step farther. When you get a voicemail, I think you should be able to hit play from the popup notification to listen to it without it having to go in the phone app.
So, summary, I think the iPhone4 hardware is perfect, it's the software that needs a bit of work.