The current hardware is already able - in theory - to become a mass storage device based system. Right now it is only working with a rudimentary serial script I cobbled together with the help of a script that Paul Stoffregen wrote to time the serial interface capabilities.
Writing doesn't even work right, I put the game data in an array in my Arduino code.
I want this to be compliant with a lot of cartridges that are out there, be just plug and play. I'm not sure how to achieve that with current skills, I'm thinking of finding (and paying) people to make this work.