On the other hand, there is one thing that could allows to expand quite easily the phone without much new developments (considering there is already a device USB present for charge and PC connection) : USB OTG (both device and host).

Is it something that is available on the PCB you target ?
If so, is it one that can power a device (like a USB storage device, a mouse, a thermal camera, ...), or one that needs a powered adapter cable ?

Obviously, I would understand if it were not available.