I don't like to say it. But I don't think that a modular design on a handheld controller is a good idea.
Especially not from Nintendo. There are a lot of different and unique ideas that I would love to see people try to innovate. But I don't think this is one of them. It also seems kind of underwhelming like a lot of other innovations from Nintendo.
I don't like it when people call the innovations that Nintendo makes bad things. But they're pretty underwhelming to me most of the time and not personally appealing. Motion control is kind of interesting, but I didn't like the Wiimote a whole lot. And 3D was kind of interesting, but it wasn't something I wanted in a handheld. And button controls are probably the last thing I really want to see modular on a handheld and from Nintendo especially.
There are lots of innovations and there's even some modular things I'd love to see in a Nintendo handheld. But it's not the controller. I'd rather see one standardized, good controller that is well made.
For instance, I'd really like a modular OS on Nintendo's handhelds. Perhaps something Android or Debian based. I also think being modular would be really nice for console controllers of any kind, especially if they were able to work for multiple systems. I'd love to be able to make my own console controller just as I would like it. That would be fun.