Realistically:

Faster processor (600 mhz or so)
No video bandwidth issues.
Transflective screen w/ 24 bit color
Dual microSDHC slots.
Keyboard that has a tab key.
FM radio
GPS (something that locks on within 30 seconds or so, although preferably it'd allow for downloading ephemeris data from the internet for pretty much instant satellite acquisitions.)
2 GB of flash is fine if they still give two memory card slots.
Double the RAM.

I'd pay $480 for THAT device, but I don't think I will for the N810.

Some of the suggestions here are absolutely crazy and would end up giving us $2000 tablets, it seems.

Also, I've seen it a ton of times here and on the easter egg post, but I don't get why everyone is so enamored with the idea of having an IR transmitter inside, I'll stick to my Harmony remote personally, does things much better than a touchscreen remote could.