I've built a new concept for the 3DS and Wii U. Game carts, and as old as it sounds, they don't just function as a game piece. The cards download the game to whatever system you specify it to download to and function as a 16GB SD card afterword. These would work almost like trading cards, owning a certain cart and trade with others. Although the games work once, they can hold game saves and run programs. These can be returned to the store and reloaded and get the price of the card back, like to keep the card would cost extra than to return it.
Maybe in years to come we will return to the classic game cart style. It would make trading and sharing games much easier. I hope Nintendo tries it out.