Well, one antagonist could be Sasuke. If he goes down a dark path again, his actions could cause Naruto great pain. Minato would then watch as Naruto tries to deal with his best friend's betrayal or attacks. Zetsu is also a potential antagonist. He has his own hidden agendas and could be manipulating events behind the scenes to make Naruto suffer. And there are always the hidden threats within the shinobi world, like certain hidden village leaders who might see Naruto as a threat and try to harm him, with Minato observing all this.
There are many out there. A really good one might focus on the emotional reactions of Team Minato. For example, Minato might be proud of Naruto's strength and how he overcomes Pain. Kakashi could be in awe at Naruto's new techniques. Their reactions can enhance the overall viewing experience in the fanfiction.
One possible theme could be regret. Minato might feel regret for not being there to protect Naruto, which adds to the emotional turmoil as he watches his son suffer. Another theme could be the cycle of pain. Naruto's suffering might be seen as part of a larger cycle that Minato was unable to break. Also, there could be a theme of growth through adversity. Even though Minato is just watching, Naruto's suffering could lead to his growth as a character, and Minato might witness this transformation.
They would be empathetic. Minato, as Naruto's father, would especially feel a deep sense of pain seeing his son struggle. He would want to reach out and comfort Naruto during his lonely times when the village shunned him because of the Nine - Tails.
Well, in fanfiction, if Team Minato watches Obito's future, they might be shocked. For example, they would witness his alliance with Madara and all the destruction he causes. It would be a moment of self - reflection for them. Minato might blame himself for not being there for Obito when he needed it most. Kakashi would also have a hard time seeing his old friend turned into an enemy.
You can try looking on popular fanfiction websites like FanFiction.net or Archive of Our Own (AO3). They usually have a wide range of Naruto - related fanfictions, and you should be able to find some with the 'Team Minato Watches Naruto vs Pain' concept.
In fanfiction, Naruto and Minato often represent a connection between generations. Minato was a great ninja in his time, and Naruto is following in his footsteps. Fanfictions can explore how Naruto lives up to his father's reputation.
I'm not sure if there is such a specific fanfiction. Fanfiction can cover a wide variety of themes and scenarios, some of which might be rather strange or controversial. However, Naruto fanfiction often explores different relationships and alternate storylines, but this particular one seems a bit unorthodox and I haven't personally come across it.
I'm afraid I'm not aware of any particular fanfic with this exact premise that I could recommend. However, when you search on fanfic platforms, read the reviews and summaries carefully. Some fanfics might have a similar concept but not the exact same title. Maybe look for ones that explore alternate universes or dark timelines in the Naruto world.
One popular theme is Minato getting to know Naruto in the afterlife. Since Minato died when Naruto was a baby, fanfictions often explore their relationship in a spiritual realm.