5. Go back to what worked (Halloween 4, Fast and Furious).
Sometimes, franchise producers get really stupid, and decide that what was appealing about their original property was neither the story nor the characters, but the general concept. Often, this is also them attempting to prove a point about stars being replaceable. So when Halloween stopped being about Michael Myers in the third movie, and the Fast/Furious series suddenly went to Japan and forgot about Paul Walker completely, the ensuing movies - whatever their merits - started with strikes against them because they weren't the continuations fans had hoped for. But the studios wised up, and both part-fours saw the return of the favorite characters and formulas. "Dinosaurs in a broken theme park on an island" is a simple enough template to not screw up, one would think.
4. Whatever the fuck they did in The Land Before Time IV (The Land Before Time IV).
"You are tearrring me APAARRRRT Dino-Lisa!"
I'm not watching to find out. But something's working for them, because there are thirteen movies in the series. Were you aware of this? That's more than Jason Voorhees has, even if you include the remake and Freddy vs. Jason.
3. The Stallone Rule of Four: skip directly to the good parts, and amp them up (Rocky IV, Rambo).
The first Rocky and Rambo films were serious movies about outsiders struggling to fit in and find their place in the world, but by the time Sylvester Stallone's alter-egos were single-handedly fighting Soviets in Afghanistan, or staring down Hulk Hogan and Mr. T, they had become cartoons. So Rocky IV gives us a robot, a Soviet steroid machine and a gratuitous death, along with some classic '80s driving-and-crying montages. Rambo is basically an excuse to have evil Asians explode into bloody showers of guts. The point Spielberg can take away here is that we don't need a lengthy set-up to get people onto the dinosaur island. Start right there, and have the first person get eaten immediately.
2. If you can't think of anything else, add 3-D (Resident Evil: Afterlife, Underworld: Evolution).
When nobody cares about your plot, but is just coming to see a tightly clad female warrior (played by your wife) kicking the crap out of monsters, the only way to really change it up is to give them a tightly clad female warrior (played by your wife) kicking the crap out of monsters...in 3D!!! Best big-budget honeymoon videos ever. JP4 will indeed be in 3D, but Kate Capshaw may not be in ass-kicking shape these days.
1. Figure out what part of your audience hasn't been served yet, and give them what they want (Star Trek IV: The Voyage Home, A Nightmare on Elm Street 4: The Dream Master).
While most Trekkers love the ideas of science fiction and the action-adventure elements of the old show, there has always been a non-insignificant section of fandom that looks forward to space hippies, Spock losing his brain, and Shatner going crazy with the hand gestures and overacting. Such fans had to grab individual moments where they could in the first three movies ("KHAAAAAAAN!") but it wasn't until the fourth that the lighter side of Starfleet got an entire nuclear-wessel-powered movie, and Spock learned how to swear. Likewise, in taking over the Freddy Krueger franchise, Renny Harlin realized that the audience was rooting more for Freddy than his victims, and played to that, making Freddy a callous, carefree quipper in the best Bond/Schwarzenegger tradition.
Has any part of the Jurassic Park audience not been served yet? It would seem to me that only a true-to-the-first-book reboot would accomplish that feat, as only book loyalists were vocally disappointed by the first film. Perhaps the other great achievement of Nightmare 4 and Trek IV should be observed more: they don't take themselves too seriously.