A few years ago, the original designers were planning on doing a Manhunter 3 that would be playable on-line. However, I doubt anyone on here can tell you exactly what happened to it.
And I believe the reason why classic Sierra never produced anymore Manhunter games had to do with the relatively weak sales of the first two.
These games were very interesting though. I guess it would be fair to consider them the predecessors of later first-person adventure games like 7th Guest (and its sequel, 11th Hour), the Myst games, Shivers, etc. Meaning, travelling through a good-looking but not very interactive (usually pre-rendered) world, solving one illogically placed logic puzzle after another.
I remember playing Manhunter: NY back in the late eighties and thinking how cool the designs were. The gameplay was however perhaps a bit too different for most when compared to what was selling well at the time.