With that in mind a Japanese blogger created this cover for a fictional game called "???????UTACO ???????????????"
http://famicase.com/11/softs/41.html
A while after a guy from Argentina took the ball and made a game with that cartridge design in mind:
YouTube Video Placeholder
(the game actually scans the microphone and reacts to what you sing)
As an classic gamer, I found this very interesting... lol
(btw, the Japanese version of BattleToads start the game with the 'A' button instead of start since Famicom has no start button on the second player controller ... lol)
NESDEV link for the game:
http://forums.nesdev.com/viewtopic.php?f=22&t=9915
(it's free homebrew and apparently work on emulators)