Inspired by Carsten’s RasPIC64 project and my experience with Tapecard SD I have created a new cartridge called Kung Fu Flash and released it as open source.
Kung Fu Flash can emulate different cartridge types, load PRG and D64 files from the microSD card and support EasyFlash 3 USB. It is what I call a Software Defined Cartridge, where a fast microcontroller emulates cartridge hardware in software.
You can find the project here: https://github.com/KimJorgensen/KungFuFlash
This is still work in progress; there is no write support, only the PAL version of the Commodore 64 is supported and the latest version of the gerber has not been tested yet.