Other than SD card, headphones jack and a couple of buttons more (select/start), I'm missing an RTC. It would allow real-time games to be played without using main battery.
I don't know if some kind of wireless communication (IR or BLE) will add to it or not, but maybe a cable is enough for connecting two arduboys, like the gameboy used to do.
But I love the minimalist design, and fear that adding too much might take the "cute" factor away from it...
I'll try to get me one asap.