LEGO mindstorm EV3 control using FPGA
You can find some information on I2C in FPGAs on fpga4fun. There are lots of I2C cores on OpenCores, too. The biggest challenge would be how to connect your FPGA to the peripherals. My advice is to get a tiny development board like this one, and then you need to supply power to it, but this can be battery-powered since FPGAs are typically low-powered.

You would need to make connectors to connect the pins to your MindStorms/PF motors, but that should be fairly easy. However, one concern would be whether the board is able to channel enough power through the pins into the motors: Lego motors are 9V whereas typical FPGA pin output is 5V.