Twilight Princess. The group needs a full-sized choir to lend its vocal talents on some of the songs, but unfortunately, hiring a full-sized choir is a super expensive thing to do.
You can find out more details about the project, and make a helpful contribution, over on the ZREO Kickstarter page. There's some sweet incentives for donors too, including a personal arrangement of your favorite Nintendo theme song. (We'll take the ending theme to Little Nemo, thanks.)