One idea could be a story where the Saiyan Blue form has an unexpected side effect. For example, it could start to drain the user's life force slowly but surely. Goku discovers this during a battle and has to find a way to either control it or reverse the effect while still fighting powerful foes.
A great plot could involve a young Saiyan who is struggling to achieve the Super Saiyan Blue form. He trains with Vegeta, who is initially reluctant but eventually sees the potential in the young Saiyan. Through their training sessions, the young Saiyan not only learns about power but also about the true meaning of being a Saiyan, and finally attains the Super Saiyan Blue form in a moment of great need.
In a fanfiction, there could be an alternate universe where the Super Saiyan Blue form is not achieved through the normal means. Instead, it is a result of a magical spell gone wrong. Goku and his friends have to deal with this new form that has different powers and limitations. They have to figure out how to control it and use it against an even greater threat that has emerged in this alternate universe.
One idea could be Pan discovers a hidden power within herself that allows her to transform into a unique form related to Super Saiyan 4. Maybe she has to go on a quest to find the knowledge to fully control this power. Another plot could involve Pan traveling to a different dimension where Super Saiyan 4 is the norm and she has to prove herself among stronger Saiyans there.
She could go on a quest to find a lost Saiyan artifact that can only be accessed by a Super Saiyan. This would take her to different planets and introduce new characters.
In Dragon Ball Super Super Saiyan 4 fanfiction, often there are concepts about alternate universes. Picture a universe where Super Saiyan 4 is the norm rather than an extreme form. This could lead to different power hierarchies and battles. Additionally, some fanfictions explore the idea of Saiyan - human hybrids achieving Super Saiyan 4 in unique ways, perhaps with abilities that are a blend of human ingenuity and Saiyan strength.
One idea could be a story where Goku and Vegeta from Dragon Ball Super end up in the world of Dragon Ball GT by some sort of time - space rift. They then have to team up with the GT versions of themselves to fight a new, powerful enemy that is threatening both timelines.
One idea could be Gohan in his Super Saiyan 4 form facing a new, powerful enemy from another dimension. He could struggle at first to control the raw power of this form while also trying to protect his friends and family. Maybe this new enemy has the ability to nullify the Saiyan's energy attacks, forcing Gohan to rely more on his martial arts skills that he learned from Piccolo.
One idea could be Ash getting transported to the Dragon Ball universe and somehow obtaining the power of Super Saiyan Blue. He could then use his Pokémon training skills combined with this new power to fight off powerful villains in the Dragon Ball world.
One idea could be exploring the hidden powers of the Saiyan survivors. Maybe there are some unique abilities that only surface in extreme situations. For example, a Saiyan survivor discovers they can manipulate time on a very small scale, which gives them an edge in battles.
Another interesting concept is a Saiyan OC who is a hybrid with another alien race. This could lead to some really cool character design and powers. They could struggle with fitting in among both Saiyan and the other alien culture, adding depth to their story.
One idea could be Goku exploring new universes in his Super Saiyan God form and facing off against unknown foes with unique powers. Maybe he discovers a new type of ki energy that only exists in these universes and has to learn to control it quickly to save his friends.
A new, self - created android by Dr. Gero's successor could be the main villain. This android could be specifically designed to target and defeat Super Saiyan Blue users. It could have the ability to absorb their energy and use it against them, making it a very difficult enemy for Goku and Vegeta to face.