There is new STM32 mcu powered low cost cartridge under development.

FORUM:
http://atariage.com/forums/topic/257488 … ontroller/

PICTURE:

Cartridge

Firmware

There is also speculation to use extra STM32 (arm) power for emulating 18MHz 6502 or as extra sound upgrade.

Now UNOcart can save into ATR atari floppy images!

YOUTUBE:
https://www.youtube.com/watch?v=dOxkTCKIIrI