So, I don't know if this is the right thread to mention this, but I can't create a new one yet, so here it goes... I found something interesting while browsing the US trademark system.
For Wii U games, you can see that in the categories for the trademark mention game discs, but not cartridges.
On the other hand, 3DS games do mention cartridges in the description:
However, for Breath of the Wild, there's something different, it also lists video game cartridges:
I don't think this is a random decision or mistake, as Nintendo has never assigned video game cartridges to a Wii U game before (unless it also has a 3DS version). However, we already know there's going to be a NX version of this game, so this could be a strong indicator that NX will, in fact, use cartridges.