Another plot could be that Harry, while being trained by Grindelwald, forms unexpected alliances with other characters who are also on the fringes of the wizarding world. They could be outcasts or those with different views on magic. Together, they might try to change the power dynamics within the wizarding world, but Harry has to constantly struggle with the fact that his mentor is Grindelwald, a known dark wizard.
Perhaps in the fanfiction, Harry is trained by Grindelwald to face a greater, unknown threat. Grindelwald could be using Harry as a tool to achieve his own ends, but in the process, Harry discovers hidden aspects of the wizarding world. This could include ancient spells or secrets about the origin of magic that Hogwarts has long forgotten or suppressed.
In the 'harry trained by grindelwald fanfic', the character development of Harry would be a key point. Harry, who is used to the values of Dumbledore and Hogwarts, now has to adapt to Grindelwald's teachings. This could lead to an internal struggle within Harry. He might question his own sense of right and wrong. Also, the plot could involve Harry using the skills he learned from Grindelwald to face new threats or solve old mysteries in a completely different way than what we expect from the original series. The fic could also introduce new magical elements or interpretations of existing magic based on Grindelwald's knowledge.
Well, it depends on what you consider 'good'. Some people might like the ones that focus on Harry's internal struggle while being trained by Grindelwald. For example, there could be fanfictions where Harry has to decide whether to follow Grindelwald's teachings or stick to his own values. However, not all such fanfictions are well - written. Some might have plot holes or out - of - character moments.
Another plot could involve Fem Harry Potter forming an unlikely alliance with some of Grindelwald's former allies who have turned against him. Together, they plot to bring him down. Fem Harry Potter has to convince these allies that she is a worthy leader in this fight, and they embark on a dangerous mission to stop Grindelwald's reign of terror.
One possible plotline could be Harry initially being forced into the training but then gradually getting intrigued by Grindelwald's knowledge. He might start to question the traditional teachings of Hogwarts. Another could be that Harry is tricked into the training, thinking Grindelwald has good intentions only to discover the truth later.
Well, perhaps in this fanfiction, Grindelwald trains Harry to be a sort of double agent. Harry would learn Grindelwald's ways so that he can infiltrate the dark wizard's followers. Grindelwald thinks he can control Harry, but Harry is secretly using the knowledge to gather information for Dumbledore. Along the way, Harry would face a moral dilemma as he has to pretend to embrace the dark arts while still holding onto his own values.
Harry might start off as a bit naive about the darker aspects of magic. But as he is trained by Grindelwald, he becomes more aware of the grey areas in the wizarding world. His character could become more conflicted as he is exposed to Grindelwald's teachings. He may question his previous beliefs and values.
One interesting plotline could be Harry somehow traveling back in time to meet Grindelwald in his prime. This could lead to a battle of wits between them, with Harry trying to stop Grindelwald from starting some of his more heinous plans early. Another might be that Grindelwald, seeing Harry's power, tries to recruit him to his side, offering him power and knowledge that Harry has to resist.
One interesting fanfiction could be about Grindelwald's early years before he became so dark. It might explore his relationships at Hogwarts and how his views gradually changed. Another could be a story where Grindelwald teams up with a young Harry Potter in an alternate universe to fight a greater evil. There's also fanfiction that focuses on Grindelwald's relationship with Dumbledore and how it affects the wizarding world in different ways.
There are fanfictions that explore the idea of Harry Potter being sent back in time and having to deal with Grindelwald during his reign of terror. This allows for a lot of interesting plot twists as Harry tries to use his modern - day knowledge to stop Grindelwald without changing the future too much.
One interesting story could be where Harry is on the Grindelwald train and discovers a hidden magical artifact. It leads him on an adventure to figure out its purpose and keep it from falling into the wrong hands.