Potential use for Euler Bus/PI-2-bus

[pfeerick]

1) What exactly can I do with the buses on the Pine64/how do I actually go about using them? 

The bus GPIO pins can be used for things like turning stuff on and off, or communication with other devices to add more functionality to the pine64 board, or to allow it to interact with tht outside world. For example, it should be possible to use a NRF24L01 radio transeiver module with the pine64 RPi bus, allowing the pine64 to receive sensor readings from a NRF24L01 based sensor network. Or it might have a series of led lights hooked up to the pine64, and you can turn them on and off via code, or have the pine64 turn them on and off to indicate different things. Think of the busses as allowing the processor on the pine64 to interact with other devices, but you need to put a bit of thinking and legwork in to make it so it can do it.

2) Would I be able to directly wire an Xbox 360 controller to one of these buses, thereby freeing up a USB port and/or the bluetooth connection?

I'm not familiar with this side of things, but I would suspect not it if it is a usb connection. If the number of ports was an issue, it would be much simpler and easier to use a USB bub.

3) How much programming/how possible would it be to modify Android to allow for controller interaction with the Android UI? For example, use the D-Pad to select apps on Android's home screen?

There is currently no support on Android for controlling the GPIO pins. I believe it has been discussed as a desired feature, and will probably eventuate, but I haven't seen anything yet indicating it is being worked on. Hopefully it will eventuate, as it would be another market the pine64 could capture, as I'm not aware of any other boards (other than the RIoTboard - and that was only limited) running Android have done this either.