Idea:
Screen is whatever resolution seems suitable/available at the time
Motherboard has up-to-date ARM SOC, TV out, internal display connection, one SDXC slot, audio out, micro USB AB, analogue volume control, power switch, USB host
Pandora weirdness includes hardware specific stuff that should be separate from the main board (e.g. battery handling)
Daughter board has one SDXC slot, audio line in, GPS, DVB/DAB, one USB host socket, solder pads for internal USB host (instead of second battery), accelerometer, microphone, random external connectors on pcb headers
Model B could be available as an upgrade from model A
The daughter board and motherboard could be improved and also made available as upgrades (or fit-it-yourself)
The motherboard could be available on its own, or in a set-top-box case, etc
I think you're onto something there - but I would count on the 'pandora weirdness' to be 1/2 of the base footprint and then each of the batteries to be just a touch lower in capacity - like 2200mAh. Then allow for 1/2 footprint modules to be stacked onto the bottom of it. Each module would be required to have both the male and female 'surface plug' on them so they can be stacked in any order.
I could see the following module types - and a case for ANYTHING that a module can do being LEFT OUT of the main motherboard/unit. Yes, this means a theoretical Pandora with NO SD slots. It may even be possible that the 'Pandora Weirdness' block above is actually just a 'wireless module'.
The leads going through the 'surface plug' would need to include USB 3.0, power, some sort of 'active battery selector' logic, HDMI leads, etc - it would need to be a high density connector. The mating between the unit and the modules would need to be extremely durable.
1. Battery module. Need piles of battery? Add two more 2200mAh half footprint bases to it = 6600mAh. No real limit here - you could stack 11 batteries on the thing and make it 4" thick with a week and a half of continuous run time if you want.
2. USB and card reader module. Need a USB 3.0 hub/card reader with two ports out the back and two SDXC slots out the front - stack one of those 1/2 footprint jobs onto it. With this as a module and this example, it may make sense to put a microSD card into the pico ITX motherboard layer in the main example.
3. Wireless Connectivity Module. Bluetooth, wifi, IRDA even? All self contained with antenna in the module.
4. Entertainment Center Connectivity Module. HDMI, RCA jacks, Stereo Mic in, Display port, anything else you can jam a plug for on any of the three exposed sides of the module.
5. Wallet Module. With just 0.5cm more depth it could hold credit cards, ID & currency. (OK - so I'm running out of ideas.)