No offense, but I think that as a concept the marketability and feasibility of a Crazy Taxi Adventure type of game expired around 2001. A family-friendly Grand Theft Auto-like from the franchise that served as a precursor to that type of open-world gameplay would've probably been a hit to the magnitude of Simpsons Hit & Run or something similar. If that planned Crazy Taxi movie had come out, especially, it might've had some legs.
At this point, gamers are far too removed from the memory of the franchise to be pulled in by a fleshing out of the world. Titles like Sonic Adventure, Super Mario RPG, Mega Man Legends, Panzer Dragoon Saga, and even in concept Shenmue/Virtua Fighter RPG, made sense at the time because the franchises were still firmly in the public mind.
The funny thing is that Toshihiro Nagoshi of Yakuza fame was a producer of Crazy Taxi, so the DNA of your idea is in there somewhere.