I had an idea for another open-world game that's set in 1970's New York City (plus Long Island and New Jersey) and it can be summed up as Grand Theft Auto mixed in with Koei's "Warriors" games.
Hell, if Koei were to make it, I'd call it Street Warriors. I'd have the characters be a mix of fictional OC's and famous real life criminals from that era like John Gotti, Paulie Castellano, Frank Lucas, Henry Hill, and the Son of Sam. Of course, they would be reinterpreted as over-the-top anime characters, same as any other historical figure in a Koei game.
Of course, Koei might need to outsource some of the work to a smaller developer given how badly Dynasty Warriors 9's open world gameplay supposedly was.