Chinatown was the first of a trilogy, but after the second film failed to achieve box office success (personally I liked The Two Jakes more), the concluding film was scrapped (plus the director isn’t allowed back in the USA).
Some characters merit a backstory or prequel, but I don’t think Jake Gittes is one of them. Who he is and what compels him to act is fully flushed out in the two films. I’d much prefer a third movie, though I suppose that’s unlikely. So I think it’s best that they forget it... It’s Chinatown.
Kevin M. (RPCV)