Jet Set Radio as well.
I imagined the game design to be like Future with more modern features. You can find fellow GG's out and about and you can start side missions to get to know them. You can earn money through skating challenges, item collecting and rival gang encounters and spend it on clothes and other junk.
I would also like to see Crazy Taxi 4. My pick for the next city is Chicago-style, around the Loop business district. The game could throw some challenge options like weather change, day and night shifts, traffic and construction zones, bridges goes up and down.