Actually thinking about it ... if they really were married to the idea of the fold-out style of the GBA/DS/3DS ... the way I guess you might be able to do it is have a large 3000 MaH on the bottom portion the handheld and another 3000 MaH panel behind the top display. That would give you a fair bit of battery power for sure.
I mean the top portion of a 3DS XL is already about as thick as a modern iPhone, so I don't think it would be that hard to fit a battery behind the display.
The way they could save money is to have a larger top screen display, but have it be able to swivel somehow to a vertical position, that way it could emulate the dual screen set up without physically having to have dual screens.
Interestingly enough I just checked and the Wii U tablet screen is exactly the same width when held vertically as the width of the original 3DS screen. I wonder if that's coincidence or if Nintendo testing that particular screen size for a reason. Still I think probably a tablet layout would be more preferable.







