Sprout, a replacement logic board for old iPods and #iPod Minis (the ones with monochrome screens), based on a Raspberry Pi microcontroller, and providing modern features including WiFi, Bluetooth, up to 2Tb SD storage, USB-C and (apparently) clients for Spotify and Apple Music. https://www.crowdsupply.com/certain-things/sprout-pi-mainboard/ They say the firmware will be bespoke and open-source. I wonder if it’ll be possible to get Rockbox working on these as well.