Our affiliate website GameCube Advanced was told this at the Pasta Record Breaking event. No details were given, so the only think we know is that the versions will link up by use of the link-cable.

What could it be? Possible hidden levels in the GameCube version, and if you want to reach those you'll need some key or weapon obtained through the GBA version. Or just extra weapons? This is all just guessing of course, but as soon as we find out more we'll tell you.

Source: GameCube Advanced