I hope Nintendo doesn't put DVD playback ability. I already own a DVD player, and adding this feature is just extra money Nintendo would almost assuredly be wasting by putting it in. If they go with a slightly different format besides Blu-Ray, I hope they up the space and not halve it like they did this Generation.
If they stick with memory cards, the sizes shouldn't be horribly limited. Maybe they could re-think what they were supposed to relase this Gen. with having the upgrade slot on a mem. card where you can insert an SD card. Or Nintendo could just make their own and maybe that would be cheaper?
I'm not a big fan of HD's built in to a console. They should just make it an add-on like Sony has/is going to whenever they get off their ass and release it in the US/Europe. Since the next console won't come out for a few years, by then prices of the "smaller" HD's should be very cheap to make and shouldn't cost too much to give to the public.
Nintendo should and hopefully will most likely put a built in Ethernet Port and possibly a 56K V.92 Port (or whatever new "thing" comes out by then).
*Edit* (Thanks for reminding me GameMaster
) Built in receivers with Wireless play all the time. And use the same power source as teh GBA SP does. (Lithium something...) Since I just heard that the new battery Nintendo will release for the SP in Japan will have something like 17 hours with the light on, you could possibly go for months without having to charge the "Wavebird 2"