Unused GPIOs, exposed on the header: IO38, IO39, IO2
I2C bus is also exposed there: IO3 as SCL, IO13 as SDA. however, since I2C is used internally, pins should remain as I2C.
BOOT pin is also on the header, so user can override it function after the application starts. care must be taken to make sure the external periphery will not mess with the boot process.
we're going to publish the schematics when we test and finalize (if needed) the design. more updates to follow.
Looks like a very useful dev board!