A mini projector attachment!
<dream>
Attaches through firewire, it projects visuals (like iTunes, except with a base of at least 30 fps.) onto a wall, or ceiling! That would be cool! Of course, realistically, it would be expensive, suck up battery power, and have a high probability of breakage, but it would be cool!
</dream>
Seriously, a color touch-screen with an OS X look and feel would be nice. Then Apple could put in a small address book, and a to-do list. They could develop it until they created the Newton all over again! Actually, the apps on an iPod would be to much. It sounds like something M$ would do. "Adjust the temperature of your fridge with a mp3 player!"
There's only so much you can do with the iPod until you have a full fledged PDA. But, then again, a PDA the size of an iPod (full screen in front) would be interesting. (can you say iWalk 2?

)
<serious>
OK, I'll sum it all up. Color touch-screen (if touch, finger taps, no stylus. But if it doesn't have a touch screen, it's okay, i'm cool with the scroll wheel) with OS X look and feel. It could also have an option to change the UI if the user doesn't like aqua. a 10gb HD would be nice too. I like the size and feel of it, because it stresses simplicity over complexity. I still think visuals would be cool, just for fun, even if they're not real fancy, it would be nice and i'm sure some people would appreciate it. Removable, interchangable face plates would be cool to, like on cell phones (knowing Apple, though, this will probably never happen).
</serious>
Oh well, I'm still on my way to iPodville!