Most people here played Jet Set Radio. Some people watched Air Gear.
So... In Jet Set Radio you can choose your Graffiti and in some new mods you can add your own graffiti. You can spray over your enemies graffiti to go through levels. So... How about a new way to gain XP: Spraying over government propaganda with your own personal graffiti that you can import from your computer just like in TF2's spray function.
How about another version:
In Air Gear to win a basic race, one of the racers need to place their sticker on the finish line. How about the same thing in Hover? For example an idea of a new mode: Graffiti Race. In this mode you'll need to race to the finish, spray graffiti in right spots and spray your last graffiti at the finish line. This will create a complicated kind of race for advanced players of the game.