I don't really see the benefit anymore. My current device lasts ~40 hours on a charge, so I seldom find the need to swap anything out. Even if I did, those little USB battery packs that charge multiple devices are more practical. On a long flight, my wife and I just share one and it works on the Switch and tablet too.
Sealed devices have way better water resistance, less plastic makes the batteries themselves bigger, and wireless charging (especially with magnets) will be challenging to add to a battery that's also the back cover.
I'm sure I'll be in the minority on this, but, I don't really have any interest in a removable battery, especially if it involves other compromises on size, capacity, and features.