This sounds amazing Gordon. Love everything in your list. Agree that Aruduio header should be left off by default but would like the ability to add header pins.
On the Pi front, one thing to watch out for is the orientation of the connector on the Pi 400. I've ended up using multiple adapters etc to get some screens and other devices pointing the "right" way around.
Also agree with Jean-Philippe on the IMU. I'm using a Pixl at the moment with an external one to pick up movement. Works fine, just messy.
On-Off switch for sure. But should totally turn off device even when connected to a charger (badge 2018 doesn't).
Would an RTC be an over-stretch? I have one soldered to one of my 2018 badges and it's dead handy, particularly if you restart the device a lot.
Are you thinking full USB on the nRF52840 or just charging?
Any thoughts on USB-C connector vs micro USB? The latest Blackpill boards and some of the RISC-V boards have moved over.
Lots of SPI Flash would be great. It's so handy on the Bangle.js
One simple thing is that any buttons should be chunkier/meatier than the tiny ones of the 2018 badge or the tack switches of the 2017 badge. Just from a user experience perspective. Also games :-)