Actually I think a better idea could be a common chassis. The common chassis could provide power, (touch)screen, buttons, audio, IMU, NFC, light/distance sensors, barometer, thermometer, cameras and leds; it could connect to the motherboard through pogopins using USB-C standard. This allows video, power and USB hub to expose the rest of the hardware to the mobo.

The mobo could have slots in the back similar to librem-5; theses slots would hold sd, wifi and modem cards.

I foresee 3 groups for whom this maybe interesting: pine64, purism and fairphone. I really think there is a market for a common platform to make linux phones competitive against alternatives on price too.

Do they make impedance controlled pogo pins for differential signals? I’m not aware of any pins that can reliably do more than high speed USB2

I'm not an EE, I really don't know... Maybe a standard USB-C could connect the motherboard to the chassis then. Still, it is possible with currently available tech.

Yes, because if there's anything PinePhone needs it's to be even more obtuse and expensive.