I believe a Jet Set Radio game for the DS could be pretty cool. The graphics lend itself to the system with using cel-shaded technique that I know the system can pull off.
The ability to design your own tags with the stylus and even trade them with friends.
When playing the game, you use the bottom screen to show you locations on a map, your info such score and stuff and even a personal Ipod like device. Basically you can scroll through music while you are playing. From songs from past JSR games to classic Sega tunes to some new songs.
Also when you go to tag, you use the stylus or your finger in the direction for the combo. So you go to a wall, before you had to actually move your anlong stick in the direction... now you actually spray the wall.
Also have support for Wirless Multiplayer including games like Tag the city... where you try to fill up the sity with tags. Or even one person controlling the police with a squad.
I think it could be a very classic game re-invented for the DS.