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.
A possible plot is that an old enemy returns and only a Super Saiyan 4 - level power can defeat it. Pan, being the young and determined Saiyan she is, tries to achieve this form through intense training with the help of her family. She could also face some internal struggles, like doubting her own abilities, but overcomes them as she gets closer to attaining the Super Saiyan 4 state.
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.
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.
One aspect of Dragon Ball Z Pan Super Saiyan fanfiction could be exploring new power - ups for Pan. Maybe she discovers a unique way to achieve the Super Saiyan form that's different from the traditional methods. It could involve her training in a secret place or having a special encounter that unlocks this power.
A fanfiction could explore Pan's journey as a Super Saiyan in Dragon Ball GT in relation to her family heritage. She might struggle with the pressure of living up to the Saiyan legacy. Her Super Saiyan transformation could be a turning point where she finally accepts her role and uses her power to protect those she loves, while also learning about the true meaning of being a Saiyan.
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.
A common theme is the growth of Pan. As a Super Saiyan, she would experience personal growth, both in terms of her power and her character. She might learn to control her temper better, which is often associated with the Saiyan transformation.
One idea could be Gohan struggling to control the new power of the Super Saiyan God. He might accidentally cause some destruction while training and then have to deal with the guilt and the fear of his power getting out of control.
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.
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.
It could be a rogue Saiyan from a forgotten part of the Saiyan history. This Saiyan might have a grudge against the Z - fighters and their descendants, like Pan. He could have developed his own version of the Super Saiyan 4 power in a more evil way and wants to prove his superiority by defeating Pan.