Jurassic Park 4 was written along with the third. After the horrible reception the third movie recieved the script was thrown out when no one would pick it up. They hired two guys to rewrite the script around 2003...they hit various speed bumps along the way and never completely finished it. I heard one or both of the writers died over the past year so the project has once again been shelved.
There were also some rumors that Michael Bay was a tentative possibility for director.
Anyway..back on track...
Agreed, LOTR can't really be considered "sequels" since all were written at once. Still a classic though..both in a literary and cinematic sense.
Most of these superhero franchises have flopped after the first movie, although I personally enjoyed the X-Men trilogy.
The SAW movies are garbage. The first one was innovative and brought a nice shock factor, they've really milked the series dry now though.
I think Pirates Of The Carribean was the last really successful true trilogy.
We'll see how the Batman franchise holds out this time around with the death of Heath who pretty much made the Dark Knight the success it was.