The big question is...
How much more technology can you *usefully* stuff into a mobile phone? Let's take each bit in turn:
1) Comms. 3G, Wifi, Bluetooth: it's all already there. Sure, the next wave of cellular tech is incoming (i.e. 4G) but 3G is more than fast enough for most people - you can download maps, stream audio/video and upload photos of your pet to youtube, all pretty much in realtime.
2) CPU: 1GHz dual-core processors are now becoming the norm, which should be more than enough to run most apps, barring some edge cases such as Apple's GarageBand or high-end games.
3) Memory: Most handsets now come with 512mb RAM (and some high-end android phones have 768mb); given that the level of multitasking you can do is generally limited by the form-factor, do you need more?
4) GPU: Most high-end handsets are using a PowerVR SGX535 (or Qualcomm equivalent): this part is DX10.1 compatible and drives Intel's GMA 500 PC chipset. This may not be quite up to the level of a high-end Nvidia/AMD GPU, but it's more than enough for the majority of games, especially considering the small screen - both Epic and iD have managed to get high-end visuals up and running on the iPhone.
5) Display: handset displays are topping out at around 4" and a 1024*640 resolution (or thereabouts). The visual benefits from boosting the screen resolution will be minimal. The only other potential upgrade is to go "3D", but it's debatable how useful this is - there's all sorts of ergonomic issues (e.g. the 3DS doesn't require glasses, but does need to be held at a specific distance/angle - and a lot of people have reported issues with headaches, etc)
6) Battery life: this definitely needs to be improved - my Desire HD generally lasts about 1.5 days inbetween charges, as I use it as an ebook reader on the train. But cramming more powerful tech into a handset isn't going to improve this...
7) Form factor: Apple are obsessed with their one-button setup and the majority of Android phones are stamped out of the "PDA with four buttons at the bottom" mould. And while most Android handsets could maybe have a bit of weight trimmed off here and there, I think Apple's gone as far as physically possible on that front.
8) Camera: boosting the resolution isn't particularly useful, given that the sensor size remains the same (i.e. more noise/poorer quality picture). You could slap two cameras onto the back to take 3D pictures, but see point 5) - and camera manufacturers have already figured out a "pan and scan" way to generate 3D images in software. Improving the flash would be nice, though...
All told, I'm struggling to think of useful new features.
Oh, and won't someone think of the developers? The Android market is bad enough - you get everything from 300mhz Chinese ePads to multi-core high-end handsets. But Apple is starting to go that way as well: between the iPhone, iPod Touch and iPad, there's somewhere in the region of 9 distinct CPU/GPU/Ram/Display combinations. Admittedly, it's probably safe to ignore everything pre-3rd gen, but that still leaves the iPhone 3GS/4, the iPad/iPad2, and the iPod Touch 3/4...