Why not the Raspberry Pi Zero? It supports OTG mode USB
The Raspberry Pi 3 is in the rejected ideas document, but why not consider the Raspberry Pi Zero? Unlike its bigger brother the Pi Zero can use OTG mode, as the USB port is connected directly, not via a USB hub.
It's very, very cheap, has handy headers to connect to, and is very hacker friendly.
See the following guide: https://gist.github.com/gbaman/50b6cca61dd1c3f88f41