Luffy is still alive and kicking in the manga. The story keeps evolving with him at the center of the action, encountering new enemies and growing stronger.
One interesting Young Luffy fanfiction could be about his early days on Foosha Village. It might explore his relationship with Shanks in more detail, like the pranks they played on each other. Another could focus on his training with Garp, showing how he overcame the tough training regime. And there are also fanfictions that imagine his first encounters with the sea beasts near the village.
In the novels so far, young Griff is alive. George R. R. Martin has left many plotlines open, including young Griff's. There are various speculations among fans. Some think he might play a significant role in the future battles and power struggles in the Seven Kingdoms. However, until Martin releases more of the story, we can't be sure if or how he will meet his end.
First, you need to understand Luffy's character well. Focus on his naivete, his big dreams, and his unwavering determination. Then, create an interesting plot. It could be something related to his early adventures or his relationships with other characters like Ace and Sabo. Also, use vivid descriptions to bring the world of One Piece to life in your fanfiction.