His strength could show up as superhuman physical abilities. He might be able to lift extremely heavy objects with ease.
He might have a tall and imposing stature, with piercing eyes that still hold a hint of his otherworldly power. His hair could be a dark color, perhaps black or a deep shade of purple, and it would be neatly styled.
She would likely have long, flowing hair, perhaps a golden color like the sun, to represent her divine origin. Her eyes could be a bright, piercing blue or amber, full of an otherworldly intelligence. She would have a graceful figure, with a certain poise in her stance that makes her seem both approachable and yet somewhat ethereal.
In his human form, Acnologia might be more complex. As a dragon, he's mainly seen as a destructive force. But as a human, he could have emotions like regret, love, or compassion. For example, he might feel remorse for the destruction he caused as a dragon.
Give the Okami in human form a backstory that ties them to their original form. Maybe they were cursed to be in human form as a punishment or they chose to take human form to better understand humans. Their actions should also be influenced by their Okami nature. For instance, they might be more in tune with the phases of the moon and act differently depending on whether it's a full moon or a new moon. Their speech could also be a bit more formal or archaic, reflecting their otherworldly origin.
Perhaps Godzilla in human form falls in love with a human. But he is constantly worried about endangering the person he loves because of his true nature. He has to battle his own kind, who see his relationship with a human as a betrayal, while also trying to keep his human love interest safe from the chaos of the kaiju world.
He might become more vulnerable. As a monster, Godzilla was powerful and feared, but as a human, he would have to face the fact that he could be easily hurt. This could make him more cautious and less aggressive.
He would probably be more impulsive as a human. Since he was always about speed as a car, he might have a hard time slowing down and thinking things through. For example, in making decisions about his racing career or personal relationships.
With Sakura, she might be initially scared but then come to see Naruto as someone she needs to protect even more. She could develop new medical techniques specifically to help Naruto in case the Kyuubi's power ever goes out of control. And she would probably be more in awe of his strength and try to learn from him how to be stronger herself.
Fanfiction often gives a new perspective to their relationship. It could be that Godzilla and Mothra have a deeper connection that was not explored in the original movies. Maybe they communicate in ways that humans don't understand, and in fanfiction, writers try to explain that connection. They might also be part of a bigger ecological balance in the fictional world created by the fanfic writers.
It often changes the narrative by making Godzilla more of an ally to humans. Instead of just being a destroyer, he becomes a protector. For example, in some fanfictions, Godzilla teams up with human heroes to fight off alien invaders.