Naruto would probably be in awe at first. He'd try to communicate with Mothra in his usual friendly and exuberant way. He might start by introducing himself and his village, just like he did with other summons. Then he'd try to understand Mothra's powers and how they can be used to help in battles or protect the village.
In this fanfic idea, when Minato summons Future Naruto, it could also be for emotional reasons. Minato missed out on seeing Naruto grow up. By summoning Future Naruto, he gets to see the strong and capable man his son has become. Future Naruto can then tell Minato about his life, his friends, and all the adventures he's had, filling Minato with pride.
In this fanfic idea, Naruto summoning Shinigami captains could be really interesting. Maybe he could summon them to help in a battle against a powerful enemy like Kaguya. The Shinigami captains' unique abilities like kido and zanpakuto skills could turn the tide of the fight. Naruto's own chakra reserves and jutsu combined with their powers would create a very different kind of story dynamic.
One possible plot could be that Naruto discovers a new and ancient summoning contract that allows him to summon Mothra. Mothra's appearance in the Naruto universe could cause chaos among the ninja villages as they've never seen such a creature. Naruto would have to learn how to control and work with Mothra to protect his village from new threats.
A possible plot could be that there is a disturbance in the balance of the Naruto world. The sages believe that only a being from another realm can help. Naruto is chosen to perform the summoning. He summons Mordred, and at first, they don't get along because of their different backgrounds. But as they learn more about each other and the true nature of the threat, they form an alliance. Mordred teaches Naruto some new combat techniques, and Naruto shows Mordred the value of friendship and loyalty in his world. Eventually, they defeat the enemy and Mordred may or may not decide to stay in the Naruto world depending on how their relationship has developed.
Well, first off, there would be a lot of destruction. Godzilla is huge and his attacks are powerful. The landscape would be changed completely. For example, if he was summoned in the middle of a battle in the Naruto world, he could easily take out large groups of enemies just by stomping around. Also, Naruto would have to deal with the fact that Godzilla is not an ordinary summon. He might not be as easily controllable as other summons. Naruto would have to find a way to communicate with Godzilla, maybe through his chakra or some other means.
The plot could be that Raven is in a dire situation in her world. She discovers an ancient spell that might be able to bring a powerful ally to her side. She decides to use it to summon Naruto. Once Naruto arrives, they face a powerful enemy that is threatening Raven's world. They combine their powers, Naruto's Rasengan and Raven's dark magic, to defeat the enemy.
One challenge is the power balance. Hollows can be extremely powerful in the Bleach universe, and Naruto has his own set of strong abilities. Combining them in a believable way is tough. Another is the lore. Naruto has its own deep lore, and so does Bleach. Keeping both consistent while merging them is difficult. Also, the character interactions need to be well - thought - out. How will Naruto communicate with the Hollows? What will his friends think of these strange summons?
A likely main summon could be Gamabunta. He's big, powerful, and has a long - standing relationship with Naruto. He could teach Naruto some really advanced water - based jutsu.
There might be a fanfic where Naruto himself has an even deeper connection with the summoned dogs. In this story, the dogs could help Naruto unlock a new power or jutsu. The fanfic could explore how Naruto trains with the dogs to master this new ability, and how it changes his relationships with his friends and enemies in the Naruto universe.