It just needs to have the base AT system board components at their usual places (which can easily be integrated into a SoC[1]) and a useful bus for expansion like PCI(e) or even (E)ISA. Video, audio, and other peripherals can be external.
The Edison module connector could've easily housed a PCI or
ISA (which slow, but more "maker friendly" because the speeds are low enough that standard TTL ICs can be used) bus, but all they put on it was USB and GPIO.
The Edison module connector could've easily housed a PCI or ISA (which slow, but more "maker friendly" because the speeds are low enough that standard TTL ICs can be used) bus, but all they put on it was USB and GPIO.
[1] https://en.wikipedia.org/wiki/Vortex86