A possible plot could be that a powerful magical artifact causes Goku and Tsunade to swap bodies for a while. During this time, they have to learn about each other's lives and powers. As they do this, they gain a newfound respect and admiration for one another. Eventually, when they return to their original bodies, their relationship has changed, and they start to fall in love. Or maybe there is a prophecy that only the combined power of Goku and Tsunade can defeat a great evil. As they work towards fulfilling the prophecy, they grow closer and fall in love. There could also be a plot where Goku is invited to the Naruto world for a martial arts tournament, and he and Tsunade meet there. Their relationship then develops as they interact during the tournament and the events that follow.
One plot could involve time travel. Android 1 travels back in time and meets a younger Goku. At first, Goku is suspicious of her, but as they interact more, they start to care for each other. Android 1 tries to help Goku in his battles against the Saiyans in this timeline. Their love story unfolds as they try to change the future for the better, with their relationship being a key factor in uniting different factions in the fight against greater threats.
There are likely some out there. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Goku and Tsunade love' and you might find some interesting stories. However, since they are from different anime universes, the fanfiction might be quite creative in how it brings them together.
Maybe Kale is sent on a mission to Earth and Goku is the one who helps her adapt. They start training together and during the training sessions, they discover they have a lot in common. Their shared love for fighting and getting stronger becomes the foundation of their romantic relationship.
Maybe Goku accidentally saves a female alien during one of his adventures. She is from a race with unique powers. She becomes intrigued by Goku's strength and simplicity. As she stays around to learn more about Earth and Goku, they start to fall in love. Their relationship could be tested by the fact that she has responsibilities in her own world.
Well, it could be about Goku Jr learning some of Goku's most powerful moves. Maybe Goku teaches him the Kamehameha. They might also go on an adventure together, like fighting a new and powerful enemy that neither could face alone.
The plot could involve Goku and Goku Black fusing accidentally. Maybe due to some strange energy anomaly. And then they have to figure out how to control this new form. They might initially be feared by their friends because of Goku Black's reputation, but then they prove that they are on the side of good by using their new powers to protect the Earth from an impending threat that no one else could handle.
Well, perhaps in this fanfiction, Goku Black makes a fatal mistake. He could underestimate Goku's ability to adapt during battle. Goku, being the experienced fighter he is, notices this weakness and uses it against Goku Black. He might use a combination of his signature moves like the Kamehameha in a new and unexpected way that catches Goku Black off - guard, leading to his defeat.
Well, the plot could involve Naruto being a huge fan of Goku through some kind of inter - dimensional viewing device. He starts to mimic Goku's training regime, much to the confusion of his friends in Konoha. Then, an evil force from the Dragon Ball universe somehow crosses over to Naruto's world. Naruto, with his new - found inspiration from Goku, decides to take on this threat, and in the process, he might even convince Goku to come and help him save his world. Along the way, there could be lots of comical moments as Naruto tries to explain ninja concepts to Goku and vice - versa.
One possible plot could be that when Goku and Goku Black fuse, they create a being with an incredibly powerful mix of Goku's pure heart and Goku Black's dark power. This new being might initially struggle with the conflicting natures within. They could then face an even greater threat than before, like a new, super - powerful villain that neither Goku nor Goku Black could have defeated alone. As the fused being, they have to learn to control their power and find a balance between good and evil within themselves to defeat this new enemy.
If Naruto was adopted by Tsunade in fanfiction, he would probably be exposed to a different set of values. Tsunade is a strong - willed and sometimes reckless woman. Naruto could pick up on her fearless attitude towards battle and also her more practical approach to leadership. Also, their relationship might change the power dynamics in the village a bit, as Naruto would be directly under Tsunade's wing.