Thank you for refreshing my mind :
Yes, of course, in case of I2C, we would need a separate interrupt, exactly as the Jolla does it (for this specific purpose. That's how the keyboard handles keypresses).


And some RGB-LED controllers. Don't forget them. They come is I2C 3 wires, SPI 4wires or some simple bit-banging 2 wires.
(+ power)


I would suggest a separate 5v input (with some power controller, just like the regular micro-USB charger port), so that it could handle variation around 5v (might happen with wireless)

and a separate 5v output (for the accessories).

Thus you could still charge and use your accessory.


Otherwise I also agree that giving USB is the best part : one could then use an embed arduino to basically do anything.


Which as you mention, up to the PWM part could entirely be covered by a chip on the USB pogopins.

For PciE / SATA : most current ARM phone chipset don't feature it (it's more often found on low-power servers and infotainment ARM SoCs : AMD Opteron 1110, Nvidia Tegra, etc.)

Though one might dream. Maybe, by the time of this potential future "ChenPhone", SATA will get popular on tons of other SoC.





Also regarding non-hacker oriented ports :

dual USB:
I've seen (mostly on Lenovo tablet) tablets featuring two USB ports, so you can charge on 1 port, and use the other port for OTG accessories. (in Lenovo, the second port is hidden behind the same cover as the microSD card)

dual microSD:
(See things as old as the tapwave zodiac, other handheld consoles like Pandora and Pyra, or some of Nokia's Nxxx Maemo/Meego internet tablets)
- the first microSD under the battery cover, for more permanent use (like on most modern phones) to store users' data.
- the second microSD external, so you can quickly and easily pop a card in for copying photos from a photo camera.

Though you have to admit that the function can easily be accomplished with an OTG reader (e.g.: meenova)
Or by straight plugging the whole camera into a powered USB hub, and optionnally implement MTP or whatever the camera uses instead of straight exposing the block-device.

Or maybe, by the time you design your own PCB, most devices will support wireless transfer (Bluetooth or Wifi) so simultaneous dual *Wifi* would be the desirable feature (like being able to connect the 5Ghz antenna to the home network, and use the 2.4Ghz antenna to copy photos over the camera's private wifi peer).