I was thinking about the requirements for the other half interface, and my bet is that USB would indeed be the best candidate. USB gives the possibility of using the broadest spectrum of devices, it's well specified, and works like charm on Linux devices.
I see the need to have both USB client mode (for computer connection) and USB host mode (for gadget connection)

However, if it is required that the interface would both source current to devices, and be able to sink current (for a contactless charging other half, for example) I think more than just pins for one USB interface is preferred.

If you just have exposed VCC, GNC, D+ and D-, it's possible to use the VC pins to both source and sink, but this requires complex (and error-prone) logic to determine what mode the pins are operating. Same way having just D+ and D+ requires backend to configure the device to work either as USB host or USB guest.

Hence I propose, that there should be 2 separete USB interfaces:

• one interface for USB host mode that can source current, maybe up to 150mA (there's battery life to consider, we don't want to drain it too fast)
• one interface for USB client mode, that can sink current from a charging solution

Alltogether that makes 8 pins, or let's say 7 if the interfaces use common GND.