Have we discussed LPDDR4 yet? That's something that might be suitable for the NX handheld as well as the NX console (if they are going the very small/low power route, as I believe they are).
These modules from Samsung are faster than the desktop variety currently available. Is the pricing prohibitively expensive?
I've been pondering what type of technologies they might decide on. My tentative conclusions are that they could go with a 14nm Soc or HBM, but not both. As people have mentioned, if they go with 14nm, it will likely be a small chip. AMD showed off a Polaris GPU and it was estimated at ~120mm2. Maybe Nintendo could squeeze a bit more surface area on there, but not much. The other point which may be relevant is how large an area the PHY for HBM requires. If each block of HBM is a 1024-bit interface, I would imagine it would require a large area on the SOC. It's too bad we don't have any die photos, but we do know that all the chips so far w/ HBM have been monsters in size.
I'm gonna go with a 14nm chip w/ 512 GCN ALUs, maybe a combo of 4 A57(or A72 we can only hope) + 4 A53, and the 32 MB pool of eSRAM. External memory would be DDR4 or lpDDR4 (if someone wants to give me some feedback on what I posted above). Thinking more and more that they could ditch the optical drive, although I still don't believe the pricing is right for Gamecards over 8 GB in capacity just yet. Looking at Wii U file sizes, not many games require much more anyway. There will be an increase in digital-only software and more NFC payment methods, especially with their Nintendo Account/ My Nintendo set in place. Look at how they've handled Wii U Digital Deluxe and Club Nintendo. They are pushing digital -- data caps be damned. Third parties support Steam and rely on digital for PC software, so if by chance, Nintendo manages to get their Assassin's Creed and CoD back, it will be a digital release (or a box in the store w/ a download code/NFC card). Also do not forget the possibility of installing games to SD Cards, which they do currently on 3DS. If your internet prohibits such large downloads, maybe you could throw an SD card in the NX handheld and find another location to download your game.
My reasoning for this is that they are going to want a very small box in order to differentiate from Xbox One and PS4. This is how I see them getting there. I'm picturing something like Wii in size, but slimmer.