Its balanced. Any more stuff to do and it would have been excessive and unbelievable for the size of the world. I'd say Witcher 3 got it just right.
It's like if you have a big meal. You say I'm full. Yeah you could foie gras style feed yourself and physically get more in but there is a...
They just brought the Witcher 3 to switch. An even bigger overworld with far more content so the limited power of the switch is no excuse to not fill the world.
In answer to the thread. BoTW's overworld was relatively large yet devoid of meaningful content. So there is definitely loads of room...