One thing I love to do when I'm driving is sing along with the radio. The problem is, a lot of rock singers go really high, and I (along with a good portion of people) just don't have that range. I always enjoy it more when the song is something I can sing, so I've always thought it would be cool to have a pitch control on a car stereo.
I think this would be a great project for TBHS. You guys could start with an existing car stereo and modify it. I'm not super knowledgable on the details, but I feel like a raspberry pi or something similar would be able to run pitch shifting software.
The key thing would be finding software that can pitch shift incoming audio, but overall, I think it would be a fun and interesting project.