Why not just keep a high-capacity SD card in that slot?
I want two full-size PCIE 2.1 slots and crossfire/SLI capability in the Pandora 2. Also pref. x86 CPU and DDR3 RAM for upgradability and software compatibility. I think that with these features the Pandora would easily replace even my desktop gaming PC
I would like to see a sdio wwan but they don't exist yet, just wifi to my knowledge.
problem with hardwiring in a 3g card directly is the incompatible wireless frequencies and standards that are used from customers of different countries and wireless carriers. Also you'd be locking yourself into a technology, say wimax/clear/lte/4g becomes a standard, you'd make it really difficult to upgrade. mini-pcie is the industry accepted standard for wwan to my knowledge.
there are some serial and usb->mini-pcie card adapters that can be bought semi-commercially, so the bus might not be a problem if you designed it yourself I'm guessing. I have no hardware hacking experience and don't even own the adapter So basically talking out my butt but it sounds like it would work.
Space might be an issue, but I'd be fine with just 1 sd card slot if it was sdxc.