I'm not following CX16 hardware development very closely, so I don't have all the details. Please correct any misconceptions.
Apparently there's a cartridge port planned, and cartridges will be able to do things that can't be done in software alone. I propose someone make a cartridge that takes an SD card as storage, and that a) allows any dev to create cart prototypes, and b) makes it easy to create copies of ROM-only cartridges.
Cartridge with SD card slot
Re: Cartridge with SD card slot
Simpler would probably be a cartridge that is all RAM, with a simple I/O slot setting of which 512KB space are read/write and which are read-only. Since there are exactly 7 possible 512KB chips, that's a single latch and some glue logic.