One common theme could be loneliness. Since Petrie is a bit different from the other characters, he might be left out or feel alone, which can be a sad situation in fanfictions. Another theme could be fear. For example, he might be afraid of not being able to keep up with his friends during a dangerous journey.
I'm not sure of specific 'Land Before Time' Petrie sad fanfictions off the top of my head. You could try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'Land Before Time Petrie sad' in their search bars and you might find some interesting ones.
One possible fanfiction could be about Ducky and Petrie going on a secret adventure. They might discover a hidden part of the valley that no one else knows about. Maybe they find a strange new type of plant or a small, friendly creature that they befriend and have to protect from the larger, scarier dinosaurs in the area.
One reason could be that the author lost interest. Maybe they moved on to other fandoms or just got tired of writing about Petrie. Another reason might be that they faced some real - life issues that made it impossible for them to continue, like getting too busy with work or school.
I'm not sure where to specifically find abandoned 'Land Before Time Petrie' fanfiction. You could try searching on some old - fashioned fanfiction archives or forums that might have such works stored away, but it's a bit of a long shot.
One common theme is unrequited love. Maybe Naruto loves someone else and Temari has to deal with the heartbreak of loving him from afar. Another theme could be separation. Due to various reasons like village duties or misunderstandings, they are forced to be apart. This creates a lot of sadness in the story.
One common theme is unrequited love. Sometimes in these fanfictions, Amy's love for Sonic is not fully reciprocated, which creates a lot of sadness and angst. For example, Sonic may be too focused on his adventures and not realize how much he's hurting Amy.
A plot could be that Ducky and Petrie find a magic stone that gives them special powers. Ducky can now speak to all animals, not just dinosaurs, and Petrie can fly super fast. They use these powers to help their friends when a group of mean dinosaurs tries to take over their territory.
Survival is also common. Since it's set in a prehistoric world, characters need to find food, water and shelter. For example, in many fanfictions, the young dinosaurs have to learn how to hunt or find safe places to hide from predators. It makes for exciting stories as they face these challenges.
One common theme could be America feeling isolated despite being a superpower. In many fanfics, it might show his internal struggle with maintaining relationships with other countries while also trying to uphold his own interests. Another theme could be related to past historical events that still haunt America, like wars or social unrest, which bring out a sense of sadness and regret.
In many of these fanfictions, a big theme is the power of time. Time can either heal the wounds of their love or make them worse. For example, as time passes, Sakura might grow more bitter about Sasuke's actions. Another commonality is the theme of hope. Despite all the sadness, Sakura often holds on to a glimmer of hope that Sasuke will come back to her. And of course, the theme of loss is prevalent. The loss of their relationship as it could have been, the loss of the happy future they might have had together.