VGA/DVI/HDMI ports have an I2C interface (though DP tunnels I2C over AUX), which is normally used to read EDID information from monitors and control brightness, but on Linux (not Windows) you can control it as a general-purpose I2C bus, dump and even rewrite EEPROMs placed on the monitor bus. In theory you could even hook up a Nunchuk or Classic Controller to a display output (with level shifters if you want to run the Nunchuk at 5V off a 3.3V bus), and write a Linux driver to communicate with the peripheral and expose it as a game controller.