homebrew should be on gc side.
unfortunately there isn't any afaik. it would be great to use a gba as a controller for wii/gamecube. retroarch would be great.
best you can do is buy a gameboy player for gamecube. it lets you use your gba for your gba, gb, gbc games. also you can use the gba for emulators on your gba flash card if you have one. NES, SMS works fine.