You should start with the media player, and the ability to display lyrics, read id3 tags with lyrics and album arts, links, etc.
A good music functionality list that needs big font/buttons is on the soundhog app. Very cool app.
Please, don't do gauges, not speedo at least, I take it most of us have a speedo/odometer, bugbyte? (What did you do with yours? LOL I got to check your install more...). If you want a track day inspired more accurate speedo you would need a 5/10hz gps that the iPad does not have?