Not sure if anyone has said this, but how about the World's Hardest Game and it's sequel? Think about how rewarding that would be. The developer of the game might be hard to contact however.
Super Crazy Guitar Hero Deluxe 5. I just got into the game, and it's really fun and addicting. Would love to see some quests added to co-enside with the in-game achievements. Plus, there are literally no music-based games with quests right now. I hope you add some in the future!