I can see the next Nintendo handheld being something closer to a original GBA/PSP/VITA in terms of design but a full on tablet would be suicide. Nintendo won't fork over the dough to have it able to do full on tablet options or the app market place to make it worthwhile.
Not to mention, while they have obviously noticed most prefer the LL/XL line for the DS/3DS, they also know that the bigger the screen the more expensive and the Nintendo portables have always been built around being relatively inexpensive for both Nintendo and the consumer. THE one time this wasn't the case was the 3DS and we see how that started off.
The design of the next Nintendo handheld won't be innovative, but it will service the needs of Nintendo and other developers. It can still be clam shelled like the GBA SP/DS/3DS and just have one true screen.







