I think it should only be done in a finale. I think perhaps a war on liberty vs. tyranny rather than Sharptooth vs. prey is something the fans have
long been anticipating.
I actually have one myself. It would be a war between a Bothtooth (or Halftooth if you prefer) and the Great Valley would be perfect.
Personally, I had it where the Leaf Eaters committed a nasty atrocity, and the Great Valley was originally inhabited by Sharpteeth. As we all know, the Mysterious Beyond isn't that hot to be in (unless you're in the Mountains that Burn. Then it's REALLY hot.

)
So they invade it. Naturally, they try and kill the Sharpteeth and bad things happen and loads die and both sides are pretty

.
The Old One, was one of those inhabitants (yeah, one of the tight parts of this story is that I intend Grandma and Grandpa Longneck and Topps to still be around in the finale, so I needed to make it so that, at least in LBT universe, dinos could live for over 100 years at least as part of their average lifespan. The Old One is 115 when she dies (and no, her death wasn't of age but by being killed, albiet accidentally, by one of the Prehistoric Pals.)
Anyway, this one would involve a start out of a war between longnecks and T-rexs and would end up with the two sides (Great Valley and Valley of Sharpteeth) having to work together to fight Red Claw (Sharptooth tyrant), Sierra (Leaf Eater tyrant), Ozzy (Egg Stealer tyrant), and Gallim (the Bothtooth tyrant and also the main villain).
The interesting part of this thing is that it would be a two part finale (like Deathly Hallows and Mockingjay, among others.) and would involve a love story too, between Littlefoot and Ali as well as other characters falling in love.
Also, Ruby would be in this movie and she too would fall in love.
Chomper, of course, you know I love him, would be in this too. He'd fall in love with a resident of the Valley of Sharpteeth and some would wonder where his loyalties lie. In fact, after the Old One in her prejudice tries to kill Chomper's girlfriend Andromeda and Chomper is forced to attack the Old One and she gets cut open and he, smelling leaf eater blood, goes nuts and nearly eats her. He then has to flee for it and he also confesses to Littlefoot that he had eaten leaf eater meat before and that he had hunted the Gang of Five in film V before recognizing them. Littlefoot's and Chomper's friendships go really dark after that, though they are restored.
All in all, this would be a totally awesome movie if I could ever get it into script and to the producers.
Note, Litltefoot isn't involved in the Great Valley conquest war as it happened 90 years beore it was born. However, in the Great Valley War, 110 years after the conqeust war, Litltlefoot, on his 25th Star Day when he comes of age, will be fighting the main villian in mortal combat in a final showdown in the Mountains that Burn.
Another interesting thing is that the villain won't be an "I'm the Villain" villain that we see all the time in Land Before Time , but will be a Prince Hans type that most don't even know is evil until near the end.
Also, another key part of the plot is that the villain will have something to do with Mama Longneck's death, and, in the final battle, tries to knock off Littltefoot's true love, Ali.
One may have wondered if this was going to be a war between Sharpteeth (specifically T-Rex) and Leaf Eaters (specifically Longneck) with Littlefoot and Chomper forced to fight against each other or end up blocking the fight like Kiara and Kovu did from Lion King II. However, the main baddies are omnivores, though all types are in heroes and villains.
Bad Sharpteeth: Red Claw, Screen, Fast Biter Leader, Thud(formerly), Chomper (very briefly), Andromeda (for a little while), Andromeda's father (for a while), Andromeda's mother (for a while), Andromeda's suitors who got mad when Andromeda chose Chomper over them.
Bad Egg Stealers: Ozzy, Strut (formerly)
Bad Leaf Eaters: Rinkus, Sierra, maybe Rhett
Bad Omnivores: Gallim (main antagonist), Odul, Ela, Finley, Liz, the Domeheads
Good Sharpteeth: Chomper, Androemda, Thud, Andromeda's parents, Chomper's parents, others who helped them fight Red Claw
Good Egg Stealers: Strut
Good Leaf Eaters: Well, most of them, minus Rinkus and Sierra, and, who knows, maybe Rhett.
Good Omnivores: Ruby and her boyfriend Ralph, also, Ptero (formerly called Arnold), a Sharptooth/Flattooth hybrid