Another way would be to have Littlefoot and the gang meet Ali as they were leaving the crash site at the end of the movie and this could serve as cliffhanger for the next movie. Instead of Invasion of the Tinysaurus we'd have something like Return to the Land of Mists with Ali having a greater roll.
This idea just made me realize something: LBT should've done what the Pokémon movies did during the Sinnoh era and made a trilogy of movies that were connected.
LBT 10:The movie begins as per usual, with Littlefoot and his grandparents leaving the Great Valley because of their sleep stories. On their way to the meteorite crash site, they're joined by herds of Longnecks; we reunite with old characters like Ali and Doc; and we meet new characters like Bron, Sue, Shorty, Rhett, etc. The rest of the gang follows behind and meets Pat like usual. The reason why Longnecks are converging and venturing out in the middle of nowhere is treated as a mystery throughout the film and is still unresolved by the end of the film; only bits and pieces of hints can be put together in this puzzle.
Because this is the tenth film, we could have cameos and/or clip show-style flashbacks of other characters and films. All the while, this new super-herd is constantly being followed by many Sharpteeth that have been habitually following the individual herds and now become occasional nuisances to the super-herd, just for action's sake. However, Chomper and his parents are among these Sharptooth, and they're not aware Littlefoot is a member of that super-herd because of its tremendous size. The film ends on a cliffhanger when Littlefoot realizes Bron is his father, just as the super-herd reaches the crash site.
LBT 11The film begins with the super-herd entering the crash site and beginning to settle in. As they do, Littlefoot deals with the revelation that he just found his long-lost father, which makes things extremely complicated with his relationships. Eventually, Bron explains why he was never in Littlefoot's life to begin with, and they begin to bond. Bron tells Littlefoot all about what his mother was once like (perhaps even finally revealing her true name), including revealing she was a capable fighter who could hold off multiple Sharpteeth (this is important for later on). He also starts teaching Littlefoot how to lead a herd of Longnecks, which gets him close to Bron's herd, including Shorty, and also sparks a romantic relationship between him and Ali. Of course, Rhett is a butt and an antagonist about it, but he eventually gets his butt kicked.
Meanwhile, the rest of the gang is lagging behind with Pat like in canon (perhaps being aided along the way by characters like Tickles and Mo), while Grandpa decides to dig deep into the mystery of the sleep stories and soon learns of the solar eclipse. The film ends on another cliffhanger, with the Sharpteeth that had been following the super-herd, an ambivalent Chomper and his parents included, gathering en masse to take advantage of the all-you-can-eat buffet they have on their hands.
LBT 12The film begins with Shorty jealously running away like in canon, and Littlefoot follows him to bond with him and get him back, but instead of being attacked by one Sharptooth, they're attacked by the super-pack. They're cornered, but before they're killed, the leader (who has been the biggest nuisance out of all the Sharpteeth in the trilogy) suddenly chooses to capture Littlefoot instead, while Shorty manages to run away. Through captivity, Littlefoot finally reunites with Chomper, who helps him escape. In the process, Chomper learns through his understanding of Sharptooth language that the Sharptooth leader was one of the Sharpteeth that Bron and Littlefoot's mother fought off in the past, and that he lost his mate to them as well; he recognized Bron in the herd earlier, saw him talking to Littlefoot, and put two and two together, so now, he's seeking revenge.
Littlefoot's escape triggers an epic battle between the important Longneck characters and the Sharpteeth super-pack, which is joined by the rest of the gang and Pat. Chomper and his parents rebel, with his parents helping Bron kill the Sharptooth leader. But before the rest of the super-pack could overwhelm the heroes, the solar eclipse happens, scaring off the Sharpteeth. The Longnecks carry out the mysterious tradition that has guided them through the entire trilogy, "saving the world". Afterwards, Chomper and his parents leave on good terms with the heroes. Littlefoot chooses to return to the Great Valley with his friends and grandparents rather than have a future with Bron and Ali, which they accept. The film, and therefore the trilogy, ends on a triumphant note, with Littlefoot recalling the life he's led so far, which put him on the path of this epic journey in the first place. Cue commemorative montage of all the films, one last scene of the gang returning to the Valley, and then...fade to ending credits.
...
Did I just write all this epicness in one go?
Anyway, back to the original point...I think that kind of approach would've solved the dilemma of Ali being out-of-place in an otherwise standalone film. Just give this premise A LOT of breathing room so she could find a role in which she's integral to the plot.
Then again, I doubt the prospect of the characters leaving the Great Valley, only to return two movies later, would be something in the crew's cards. XP