They could release a clamshell design with Joycons right on the bottom side that slide out if they really wanted to, but I think people will be fine with the tablet form factor and even prefer that to the old clamshell design.
Clamshell is not the be-all, end-all, Nintendo made portables for damn near 14 years before they ever made a single clamshell designed handheld and sold millions and millions of Game Boys before the GBA SP. Too many kids here think clamshell = Nintendo portable ... no kids, there was plenty of time before the GBA SP/DS/3DS when Nintendo made portables with a different form factor.
When I was growing up you sure as hell could not put your Game Boy in your jean pockets.
Beyond that, tablet is the more popular form factor today.
