One idea could be that the ponies somehow travel to the Land Before Time world and have to learn the ways of the dinosaurs there. They could teach the dinosaurs about friendship and magic in return for learning survival skills from them.
A plot could involve a young pony getting lost and ending up in the Land Before Time. There, a kind - hearted dinosaur befriends the pony. Together, they embark on a journey to find a way for the pony to return home, all while facing various challenges and learning from each other's cultures.
One idea could be that Twilight Sparkle accidentally activates a time - travel spell and ends up in the past, right when Equestria was first being founded. She has to figure out how to get back while also making sure she doesn't change history too much.
Perhaps there could be a magical element in the fanfiction that causes the transformation. A strange comet passes by and its energy alters the DNA of the dinosaurs. As a result, they start to transform into hybrids of different species. This would lead to a lot of confusion and also new opportunities for cooperation and conflict among them.
One idea could be Ducky getting separated from the group during a big storm and having to find her way back. Another could be Ducky making an unlikely friendship with a new dinosaur species that the others are initially afraid of. Or perhaps Ducky discovers a hidden place in the valley that has special plants that can heal any injury.
One idea could be an OC who has a special ability to communicate with a previously unknown species in the Land Before Time world. This could lead to new alliances and adventures for Littlefoot and his friends.
One idea could be an OC that is a new type of dinosaur species that has unique abilities, like being able to communicate with certain plants. This OC could then help the main characters in their adventures, perhaps by finding a hidden path through a dangerous area.
One plot idea could be Bron leading a group of dinosaurs on a journey to find a new watering hole during a drought. Another could be Bron having to protect his family from a new and dangerous predator in the land. Maybe Bron discovers a hidden valley full of strange and wonderful plants that could change the way his herd lives.
Another plot could involve a human who is a scientist from the modern world accidentally traveling back in time to the 'Land Before Time' era. This human could try to study the dinosaurs' behavior and in the process, discovers a way to communicate with them on a deeper level, leading to some heart - warming and adventurous interactions.
One idea could be exploring the relationship between Littlefoot and his mother more deeply. Maybe write a story where he has dreams that give him more insights into her. Another idea is to create a new adventure for the gang where they encounter a completely new type of dinosaur species that has unique behaviors and abilities. A third idea could be to write a fanfiction set in the future of the Land Before Time world, seeing how the characters have grown and how their society has changed.
Another option could be Rainbow Dash as a main character from My Little Pony. She is bold and adventurous, always ready for a challenge. In the Land Before Time, Spike could be a main character. Spike, being a dragon in the pony world, could have an interesting connection with the dinosaurs. He could share his knowledge of different species (from his experience with ponies and other magical creatures) with the dinosaurs, while also learning from them about their ancient ways.
One idea could be a pony being reborn with memories of a past life in a different pony world. They could use that knowledge to bring about positive changes in Equestria, like finding new ways to solve friendship problems.