Naruto and Kirby would be really interesting. Kirby could copy Naruto's abilities, like his running speed or his ability to use chakra. And Naruto could be amazed at Kirby's cuteness and his unique way of fighting, like inhaling enemies. They could have a really fun dynamic together.
However, you can't discount characters like Kirby in this context. Kirby has the ability to copy the powers of his opponents in the Super Smash Bros games. If he were to face Naruto characters, he might be able to copy their jutsu and use it against them. Also, his floating ability and quick attacks make him a formidable foe. So, it really depends on how the author of the fanfic wants to balance the powers and abilities of the different characters.
It's likely that Naruto himself could be very powerful. His ability to use chakra and perform powerful jutsus like the Rasengan and Sage Mode would give him an edge. He could also use his shadow clones to confuse opponents.
It's likely Goku. His Super Saiyan forms give him incredible power, speed, and energy attacks. He can constantly increase his power level and has a lot of combat experience from his battles in the Dragon Ball series.
Goku would likely be one of the most powerful. His ability to constantly increase his power level through training and his Super Saiyan forms give him an edge. He can also use his ki attacks at a very high level. However, Kirby could also be a strong contender. He has the ability to copy the powers of his opponents. If he were to copy Goku's power, for example, he could become extremely powerful. Another powerful character could be Meta Knight. His swordsmanship and flight abilities combined with his strategic mind would make him a formidable foe in this crossover.
It could be Naruto in his Nine - Tails Chakra Mode. He has immense power and speed, which would give him an edge in the Smash Bros battles.
Another idea is that the Smash Bros characters are transported to the Naruto universe. They'd have to adapt to the ninja world. For example, Mario might try to use his jumping skills on the rooftops of Konoha, while Link could use his swordsmanship against ninja foes. It would be really interesting to see how they interact with the Naruto characters and learn about chakra.
However, Saitama from One Punch Man (if we were to include him in this crossover fanfiction for the sake of comparison) would be extremely powerful. His one - punch ability means he can defeat almost any opponent with a single strike. But in the context of My Hero Academia characters only, Todoroki might also be a strong contender. His ability to control ice and fire gives him a great range of offensive and defensive capabilities in the battles.
Well, a really engaging plot point often involves Naruto bringing his unique ninja techniques into the Smash Bros arena. For instance, his ability to use chakra in a world full of characters with different power sources. Also, the idea of Naruto trying to understand the Smash Bros power - ups and how they can enhance his abilities or those of his newfound allies. And then there's the whole concept of Naruto facing off against characters with completely different moral compasses than his own, like some of the more villainous Smash Bros characters.
Goku would probably be the most overpowered. His ability to constantly increase his power level and his wide range of energy attacks like the Kamehameha would make him a tough opponent in Super Smash Bros. He can also fly, which gives him an advantage in the game's arenas.
Aphmau from MyStreet and Freddy Fazbear would be an interesting combination. Aphmau is a strong - willed leader in her world, and seeing her face off against Freddy, the iconic animatronic from FNAF, would be exciting. Another interesting pair could be Aaron from MyStreet and Foxy. Aaron is often protective, and Foxy has that unpredictable nature.