Lots of ideas... Here are mine -
1. Wireless networking, standard - How cool will it be to take you GC to your neighbor's house, and you only have to hook it up to a TV, and your read for LAN gaming? Even better, put a base station so that both houses are in range, and you can do LAN without unhooking your N5 from your TV.
2. Wireless networking (perhaps just bluetooth?), on the next GBA - Makes interfacing N5 with the next GBA quick and painless. Include a boot disc with every GBA (GBA games + emulator should easily fit in N5 RAM) and then you'll really have a reason for people who buy a Ninty handheld to buy an N5. This raises serious possibility of pirating games, though. As long as the wireless network is properly encrypted, it shouldn't be a problem...
3. Hard to pirate medium - I don't care what, I don't care how, as long as it's cheap to mass produce, and difficult to pirate.
4. PVR capabilities - Ninty missed the boat on DVD's, and the $20 DVD consortium fee won't be worth it next gen, IMHO. PVR (think TiVO) capabilities would be a big selling point, and is something Sony and MS are probably going to do. Nintendo will be in trouble if they miss the PVR boat. This will necessitate a hard drive.
5. Easy to develop for - It didn't help Ninty this gen because of Sony's head start. It could be an advantage next gen, though, if the dates when devs receive kits are close enough. Sony, fortunately, looks to be leaving this door wide open with the Cell initiative.

6. Easy to develop for* - Keep improving developer relations!
7. *Wishful Thinking* Dual read head disc drive - I don't know how practicable this idea is, but the potential for doubling bandwidth (and thus potentially halving load times) is mouth watering.
8. Optional Base Station - For hooking the wireless network to the internet. Can either do two flavors (56k modem and broadband) or combined. That, or just sell a 56k modem base and build in an ethernet port.
9. Mature Brand - Nintendo's kiddy friendly image is an asset, despite some opinions to the contrary. Nintendo needs to do what Disney did with Touchstone pictures, and make a mature brand that appears to be independent of Nintendo. This may also require that Ninty remove the explicit name branding of it's console (not sure how I feel about this one, honestly, because it could hurt N5 sales). (credit where credit's due, someone brought this up in another thread, but I mentioned it years ago).
10. Backwards Compatibility - This is a must if the lifespan of the NGC will be as short as rumored. We don't what the loyal base feeling burned, especially any late adopters.
11. Portable LCD Screen - probably too pricey to include, but would be nice to have this accessory available from the get go.
I don't think that I've listed anything that is too far out there.
BlackGriffen