The World Government could become the main antagonist. They would see Luffy's new power as a threat, especially if he starts using it in ways that oppose their rule. They might send out their strongest agents, like the Admirals, to take Luffy down. The Admirals could have developed new techniques to deal with the dark dark fruit powers, making them a formidable foe for Luffy.
However, in the world of One Piece, it's been said that eating two devil fruits would cause a person to explode. But in fanfiction, this rule might be bent. Another consequence could be that Luffy's personality changes. The new devil fruit power might come with some sort of consciousness or influence that alters his decision - making or how he views the world. He might become more aggressive or more cautious depending on the nature of the new power.
In such a fanfiction, perhaps Luffy's character would change drastically. The dark dark fruit might give him new powers like controlling shadows more intensely than Blackbeard. He could use it to create shadow clones of his crew for strategic battles, adding a new and darker dimension to his fighting style.
Well, Goku Black could be the more dominant character. His dark nature gives him a different kind of power. He doesn't have the same moral constraints as Goku. This could allow him to make more ruthless and effective decisions in battle. He might be able to outmaneuver Goku if he plays his cards right.
Goku in Ultra Instinct mode is extremely powerful. He has superhuman speed, strength, and can dodge attacks effortlessly. However, Luffy also has his own set of amazing abilities like his Gear Fourth form which gives him enhanced speed and power. But I think Goku would win because his combat skills and power level are just on a different scale compared to Luffy.
One interesting plotline could be Luffy struggling to control the new powers at first. Since Logia fruits give the user the ability to become an element, Luffy might accidentally turn into his Logia form at inopportune times, causing chaos among the crew. For example, if his new power is related to fire, he could set the sails on fire by accident while trying to learn how to use it.
One common theme is friendship. Since all these characters value their friends highly in their original stories, in fanfiction they often form strong bonds across their different universes. For example, they might fight together against a threat that endangers all their worlds.
It's hard to say. Goku has achieved many powerful forms like Super Saiyan Blue and Ultra Instinct. But Fasha was a strong Saiyan in her own right. If Fasha has been training hard in the fanfiction, she could give Goku a tough fight. However, Goku's experience in battles against strong foes like Fasha could also be a determining factor.
It's really hard to say. Goku has the ability to reach incredible levels of power through his Super Saiyan forms and can destroy planets. However, Naruto has the power of the tailed beasts and his Sage Mode which also makes him extremely strong. It would depend on the situation and how the writer wants to balance their powers.