the system will become top heavy if you use buttons/dpad
clamshell is dead now that the slider has been introduced. If Nintendo is smart they'll figure out that everyone's phones make it so portability is no longer wanted. Then again they invented the XL models so I think they were onto it all along.
Clamshell is not so much for portability anymore as it is to protect/childproof the screens. What Nintendo could do is introduce something like an IPad Smart Cover for their next handheld. In other words it would be a clamshell like overlay for the system that could be made of rubber or fabric and when you want to play you just flip it up and behind the system and then it would double as a grip. But aside from that the system would be shaped like a 2DS, Vita, or WiiU.