I've done a little bit of study on this stuff, I'll see if I can find my notes.
I hate notes. x_x The town map can obviously be hacked, but what about the phone? What about the times that people call you and at what rate do they call? Another interesting thing that could be hacked is comparing the RTC and showing the time in the Pokenav. Also, how about being able to extend the list so it can scroll? It could show that there would be more features than what the original games had to offer. I would like to see a hacked Pokenav and maybe have one in my hack if this is done, and change to Emerald from FireRed. Also, (EDIT) what about making it so you can place markers on the map and plae custom notes for your travels like in Heart Gold and Soul Silver?