I think Apple should stop making it thinner. I bought a case to make it thicker since it was just slipping out of my hands. And it is just... too thin to hold it in both hands (landscape).
Realistically, I'd like, in order of preference:
1. GPS + Magnetometer. Come on Steve, it's not like putting a GPS unit and a Compass inside the iPod Touch is killing the iPhone. This would actually boost sales... because we all want a GPS. Or most of us do.
2. Fix the camera. It's not broken, but please, even if I don't want a dSLR quality camera (since I'm perfectly happy with my real camera), at least make it something useful. Sure 720p Video is useful, but 0.93 MP? If you want to make this a camcorder, then take out the stills feature.
2MP camera, Fixed-Focus, Backside Illumination is a bare minimum.
Something which would make me sell old iPT4 and buy a 5th Gen one would be a 3MP Autofocus (3GS Quality, and add Backside Illumination) one. Flash (Camera) is optional, but it'd be awesome. I'd hate carrying my dSLR everywhere to take /decent/ pictures in the dark.
3. Vibration Engine. Does it really cost /that/ much?
Unrealistic things, in order of possibility, most realistic first:
1. Better Speaker/Mic.
2. IPS Retina Display, iPhone-4 Quality.
3. Mute Switch.
----- Unrealistic Line below:
4. Non-Scratchy Back
4.1. (Candidates) iPad-like Anodized Aluminum Back
4.2. iPhone 3G/3GS like Plastic Back (Two generations old... bring it in.)
4.3. Aluminosilicate Glass (iPhone 4-like), unrealistic, definitely not coming unless iPhone 5 is going to be a revolution and brings an even better redesign
5. Redesign.
5.1. Change the back.
5.2. Introduce a White version. (Now I'm just dreaming)
5.3. Shift the headphone jack to the top (This has bothered me)
5.4. Make it thicker, and add more features
I won't be buying the 5th Gen unless it is a revolution anyway. And it definitely won't be, it'll probably be something like iPod Touch 4GS. Make it faster, thinner (Really, I think Apple will shrink that thing to 32 nanometers and then realize we can't hold it anymore), and so on.
Ah well.