This is exactly what I think.
I was a bit deceived seeing that only I2C would be availlable as it kills most of the "advanced" uses of the other half.
If the only way to transfer enough data is wireless, then it is more interesting to make an independant device: it allows to continue to use the device remotely, doesn't need to change the other half, and works with all phones !
The only benefit of the other half in that case would be to share the power source, but again, if the cost is to lose the wifi connection when using the other half... (there are chances the other half will provide its own network, which is separate from your house/work one).
If some of (HDMI, DSI, CSI, SPI, USB, SATA) was availlable, there would be interest to embed some parts in other half, but with I2C, I can only see the keyboard and bigger batteries other half that make sense.

I had at first the idea of a CANbus other half, to be able to diagnose some controllers (automotive or industrial) using only the phone, but this would need at least an SPI bus running at about 10Mbps. So the only solution I have now is to make a CANbus/wireless gateway, and I don't see anymore why embedding it in a other half, as the wifi allows monitoring the system in a 100 meter range without any wires, and works with any laptop, tablet or mobile device...

I am still waiting for Jolla to see what they will do with the other half, as for me, most of my ideas went away when the I2C only connector was disclosed... Still, the keyboard will be a good one.