I like the idea of a light gun shaped like Fox's blaster. A Samus full arm cannon also sounds like a cool idea, but I have a feeling the novelty would wear off pretty quickly on that one and you would be left with an expensive and unused piece of kit rather quickly.
But the point is all mute anyway. I would guess there is about a 2% chance that such a device will ever be released for GameCube, and that is being generous. Such devices raise plenty of moral issues, especially in this day and age. Like robofish said, "light gun games aren't THAT popular", and it is something that Nintendo will simply avoid.
If you want to play such games, get a PS2.
Chris.