One possible plot twist could be that Oliver goes back in time and accidentally changes something that makes his friends turn into enemies in the present. For example, he might prevent an event that was crucial for the formation of their friendships. Another could be that he meets a younger version of himself and has to convince that version to make different choices without revealing too much about the future. It could also be interesting if he goes back to a time when his city was in a different state, like before the rise of a major villain, and he has to stop that villain from ever emerging in the first place.
Oliver can start by learning the language of the time. If he goes back to a different era, say the Middle Ages, understanding the local language would be crucial for communication. He could also study the social norms. For example, if he's in a very hierarchical society, knowing how to behave around different classes is important. Another way is to get used to the different living conditions. If there are no modern conveniences like electricity, he has to learn to do things the old - fashioned way.
His archery skills would be just as sharp, but he might not have access to the high - tech arrows he has in the present. In the past, he'd have to rely more on basic, handmade arrows. His combat skills would also be a bit different. He's learned a lot from his battles in the present, so going back in time, he'd be more experienced than the local fighters. However, he might not be as familiar with the fighting styles of that era, so he'd have to adapt quickly. And his survival skills, which he's honed over his years as a vigilante, would be a great asset, but he'd have to use them in a different context, like dealing with more primitive living conditions.
A typical plot device is the discovery of a time - key object. In Oliver goes back in time fanfiction, this object could be a magical amulet or an old - fashioned watch that allows him to travel through time. This gives a clear reason for his time - traveling. Another plot device is facing a villain from the past. Oliver may have to outwit a powerful historical villain who is causing chaos in that era. And cultural shock can be a plot device too. When Oliver arrives in the past, he has to adapt to the different customs, languages and social norms, which can lead to some humorous or tense situations.
She could meet her younger self. This would be an interesting encounter as she could offer advice and guidance to her younger version. Maybe she could help her younger self to be less afraid of her powers.
One idea could be a story where Oliver Queen finds himself in Kara's universe by some sort of multiverse accident. He teams up with Kara to fight a common enemy that is causing chaos in both their worlds. Their different fighting styles, Oliver's more grounded archery and hand - to - hand combat and Kara's super - powered abilities, would make for an interesting dynamic in battles.
One great 'dark Oliver Queen fanfiction' is 'The Dark Archer's Legacy'. It delves deep into Oliver's darker side, exploring how his experiences on the island could have made him more morally ambiguous. Another is 'Shadows of the Hood', which shows a more ruthless Oliver in his crime - fighting activities, with a lot of complex character development.
First, research the Once Upon a Time universe. Know the characters of Regina and Emma well. Decide on a time period to send them back to, like the early days of the Enchanted Forest. Then, think about how their relationship would be different in that time. Start with a strong hook, like a mysterious spell that transports them.
He could change the course of major events. For example, he might prevent the rise of a powerful villain. This would have a huge impact on the entire galaxy. Maybe he could save some characters who died in the original timeline.
One interesting aspect could be that Luke goes back to a time when the Sith were just emerging. He could try to root out the Sith threat before it became as powerful as it did. He might have to be very careful not to disrupt the timeline too much, but his Jedi wisdom could be a great asset in this time - traveling adventure.