I like the idea of a 6 Air (5"-6" screen to capture the entire Phablet range up to iPad Mini). I wouldn't buy one, but it would allow for a significant difference between the current form factor (with as big a screen as they can fit into it) in the marketplace.
Depending on how things have improved, we could prob get to 4.2/4.3" in the current 5S case size.
Decent upgrades to clarity, lens quality, stability etc for the camera would be excellent. I still hate the low light issue, but even pro-cameras require decent flash to get the quality we're all looking for. HDR could also have the ranges improved dramtically, perhaps the option to customise the number of exposures used could be good here (replacing, or augmenting, the 'live filter' crap everyone has in everything now).
Build quality is the next one for me. The 5 was shocking, I've literally shaved all the edges on my phone by a good 2-3mm now. I haven't seen the 5S in long enough use to see if that stayed an issue or not (in the Space Grey model anyways).
For iOS 8, I think this next step is going to be dragging out as many core service API's as possible. I want to be able to set defaults, enable passcode per app etc etc.. There are core settings in general that should be made into services that any third party app can use. That would be killer for me. This includes Siri... Spotify Siri intg would be perfect, "Open Spotify and play songs by Mansun" ... I think the scope for that in iOS8 is massive and shouldn't be discounted as a potential game changer (giving the android folk something less to shout about haha)
I think the rest of it is just going to be 'growing' up from the 'Hot' colour ranges. We've seen some of this being addressed already in 7.1, but I think it can all be done in a more grown up way. I find that choosing a background really does affect my enjoyment of iOS7 and how the interface as a whole picks up the colours etc. Perhaps the way to work with this further is to have the white point set by the choice of background, subtly adjusting the colour width in use.
From a tech point of view, I'd defo like to see contactless charging OOTB, as well as NFC (with additional passcode/touchID security options). I think speaker quality could also be greatly improved for this generation, perhaps the speaker glass has been refined enough now that it could be used in phones and we can do away with speaker ports on the top of the phone screen (giving more real-estate to play with).
I think I'd like to see more status on the edges on the phone as well, if I'm in silent mode, enable more stuff around the phone so I can see whats going on without looking at the screen. There was a great concept for this edge screen stuff a while back, but I'm thinking something much subtler. I have a nice glowey sleeping thing on my mac, that just shines through the chassis.. more of that please
Anyways, I've rambled enough now... I'll just leave that along with the rest of the suggestions in this thread.