I could see them doing it with or without detachable Joycons.
It is just a matter of what choices they make.
Joycon only games use separate Joycon either way, its just a matter of if they are stowed and charged by the handheld unit or not.
I think the clam shell design makes the most sense for portability and compatibility, but the joycon-less design would obviously be the cheapest manufacturing wise.
Maybe they have more imaginative engineering, who knows, but I think the goal whichever way they go is to have this new model ready for next year to launch at $199 along side Pokemon 2019 or Animal Crossing.