The plot might involve the grandson having a secret heritage that links him to both Kaido and Big Mom. This could be a source of great power, but also a great danger. He might be hunted by other pirates or factions who want to use him for their own gain. In the fanfic, he could start off as an underdog, but gradually grow in strength and influence, all while trying to keep his true identity a secret from Kaido and Big Mom.
To develop the grandson character, his goals and motivations need to be clear. If his goal is to learn from Kaido and Big Mom to become a great pirate himself, then his actions would be focused on impressing them and getting their teachings. On the other hand, if he wants to take them down, he would be constantly plotting and finding ways to undermine their power. His growth throughout the fanfic could be shown through his changing attitudes towards power, friendship, and loyalty. For instance, he might start off believing that power is everything but then realize the importance of loyalty and friendship through his experiences with different characters.
As of the latest chapters, Big Mom did die. The circumstances surrounding her death were quite complex and tied to a series of intense battles and plot developments.