Someone is doind what Nintendo Should have done long time ago with Switch
Hellz yes to Hall effect everything.
As for the rest, the modular setup is a cool idea, but it plays much better on a fully customizeable platform like PC than it would in a console. Consoles, for better or for worse, count on a standardized experience for both hardware and controls. Nintendo would be better off creating reversible joycon with an analogue stick on one end and a touchpad on the other (which would, as a bonus, give every player a form of dual analogue in single-joycon play, not that Nintendo cares about that).
WITH THAT SAID, to more or less completely contradict myself, Nintendo does have a patent (
https://www.freepatentsonline.com/20160231773.pdf) filed way back when "NX" discussions were a thing that would allow them to implement a form of dirt cheap swappable controls without any electronic components. The patent basically covers a device using internal cameras/sensor to track reflective components in a completely non-powered attachment and interpret these as game inputs. Back in the day, a lot of people assumed this was how the Joycon would work, but I think the only things Nintendo really used this for was Labo and Ring Fit.
All this to say, Nintendo could potentially use a camera or a linear image sensor like the one they already have in the joycons to track the internals of cheap, swappable components. Buttons, joysticks, or even a "touchpad" should be possible if they do it right. There might be limits on the technology--too inaccurate, adds too much latency, etc.--but, in theory, there is nothing stopping them from pursuing similar functionality to the AYANEO 3 in a very Nintendo way (i.e. using withered technology + a sideways approach). It would also allow for much more custom replacement hardware from 3rd party accessory manufacturers and users with 3D printers and an imagination, which, sadly, is probably the real reason they'd never want to do it.