Well, it could be about Broyl and Goku having to face various threats on Earth together. Maybe they start off as rivals but then realize they need to team up to defeat powerful villains. For example, they might encounter an alien race that wants to take over Earth. Their combined strength and different fighting styles could make for an exciting story.
In a Naruto fanfiction where Goku is sent to Earth, there are many possibilities. Goku's arrival could change the power balance completely. He could introduce new ways of fighting to the ninja world. For example, his Instant Transmission could be a game - changer in ninja battles. Also, his ability to constantly increase his power through training might inspire the ninja to train harder. His presence might also attract the attention of powerful enemies from other dimensions who sense his great power, leading to some epic multi - dimensional battles.
It could be a powerful demon from another dimension. This demon could sense the power of Broyl and Goku and see them as a threat to his plans of universal domination. So, he decides to target Earth, where they are, to try and eliminate them.
If Arya is sent to the past, she might first try to find a way back to her own time. But in the meantime, she could end up influencing key historical events. Maybe she'd get involved with the Targaryen rule and change the course of their downfall. Or she could befriend some of the great warriors of the past and learn new fighting techniques that she could bring back with her if she ever returns to her own time.
One interesting plot could be Naruto using his future knowledge to prevent some tragic events in the past. For example, he might stop the death of certain characters or change the course of a big battle.
One big challenge would be not being believed. People in the past might think he's crazy when he talks about future events. Also, he has to be careful not to change too much and disrupt the whole timeline. Another challenge could be that some of the villains in the past might be more powerful than he expected, and he'd have to find a way to defeat them with his current skills and limited knowledge of the past.
One common theme is family discovery. Harry gets to know his family in a way he never could in his own time. Another theme could be time - related challenges. He has to deal with the rules of time travel and not mess up the timeline. For example, he might have to be careful not to introduce future magic that could disrupt the past.
One possible 'sakura sent to past fanfiction' could be about a girl named Sakura who is magically sent back in time to a historical era. She might use her modern knowledge to change things in the past.
She could change the fate of her family. Since she knows what will happen in the future, she could stop the execution of her father, Ned Stark. This would have a huge impact on the political situation in Westeros. The Stark family might gain more power and influence, and the War of the Five Kings might not even happen or at least take a very different form.