In some fanfictions, when Harry is alone on the train, he could be transported to an alternate dimension or time. He might meet different versions of his friends or even his parents. This would be a really cool story as he could learn from these encounters and bring back valuable knowledge to his own time and world.
One interesting story could be about Harry facing challenges alone in the Forbidden Forest. Maybe he discovers a hidden magical creature that no one else knew about and has to figure out how to deal with it on his own.
One interesting concept could be the Hogwarts Express being enchanted in new ways. For example, it might have the ability to travel through time as well as space. Another could be the idea of secret compartments on the train that are only accessible to certain students with special powers or knowledge.
One interesting story could be where Harry is on the Grindelwald train and discovers a hidden magical artifact. It leads him on an adventure to figure out its purpose and keep it from falling into the wrong hands.
Obviously, Harry Potter himself is a main character. Since it's about him being alone on the train, he is the central focus. But sometimes, other characters like Dumbledore might make an appearance in his thoughts or as a sort of guiding force. For example, Harry could be reminiscing about Dumbledore's advice while on the train.
Maybe in the fanfiction, Harry uses the Time - Turner to train with long - dead great wizards. He travels back in time to learn from Merlin himself. Merlin could teach Harry ancient and powerful magic that has been lost over the centuries. This would give Harry an edge in his fight against Voldemort and also expand his understanding of the wizarding world's history.
One possible storyline could be that Harry discovers a new form of magic related to time manipulation. He then has to train new wizards on how to use it carefully to avoid disrupting the wizarding world's timeline.
One interesting aspect could be Harry's self - discovery journey. In many 'harry is alone fanfiction' stories, he has to rely solely on himself, which forces him to dig deep into his own capabilities. For example, he might learn new spells on his own, without the help of his friends or teachers. This makes for a very different Harry than the one in the original series.
One possible plotline could be Harry exploring his magical abilities in solitude. He might discover new spells or magical artifacts that he keeps to himself. Another could be him building a secret sanctuary where he can retreat from the wizarding world's chaos.
Perhaps the Weasley twins could get into a situation where they're home alone at the Burrow and decide to play some elaborate pranks on any unwanted visitors using both their wizarding skills and some ideas inspired by Home Alone. They could charm objects to move on their own and create illusions to scare off people.
There are fanfics where Harry is alone and has to face a time - travel mystery. For example, he might find himself suddenly sent back in time to the days of the founders of Hogwarts. Alone in this new (but old) time period, he has to figure out how to get back while also dealing with the different magical practices and politics of that era. It's a really exciting concept as it combines the magic of 'Harry Potter' with the mystery of time - travel.