Another thing I am considering using the SPI link from the Rpi to the Ice40 for, is audio or video data. So you can decode an MP3 on the Raspberry Pi and send it to the FPGA for processing and then send it to a 1-bit DAC and a speaker, or to a more sophisticated audio device. The fpgafun site has a 1-bit DAC example doing something like that.
I posted a video that was doing a similar thing with video data using the icoboard. That allowed either an mp4 video or a camera feed to be sent to the device that then displayed via an Oled Pmod. It would be nice to do a similar thing with a Gameduino 3 connected to the STM32, but as that is also an SPI device it might prove tricky.
Yet another possibility is to use the Raspberry Pi as an HDMI adapter. So I could send video data from the Ice40 over SPI to the Raspberry Pi and then display it on a TV via the HDMI connector.
Or you could combine these things and use the Blackice as an image processing device sending data from a Raspberry Pi camera to the Ice40 over SPI and returning the processed data over the same SPI connection and sending it to a HDMI device.