Goku from Dragon Ball could be an exciting character to include. His superhuman strength and fighting abilities would be a great contrast to the more normal - powered characters in Summer Wars. He could train the Summer Wars characters to be stronger or help them in battles against extremely powerful foes. His positive attitude and love for fighting strong opponents would bring a different energy to the crossover story.
Harry Potter would also be a great addition. His knowledge of magic could be combined with the digital world of Summer Wars. He could use spells to protect the virtual world or help the characters understand new magical elements that might appear in the crossover. His loyalty to his friends and his courage would also be traits that would fit well with the Summer Wars characters.
A character like Alice from 'Alice in Wonderland' could be really interesting. She's used to strange and magical worlds, so she could interact with the Invisibles in a unique way. Maybe she could help Arthur navigate some of the more confusing aspects of the Invisibles' world with her own experiences in Wonderland.
One interesting character to include could be a brave knight from a medieval - themed world. The contrast between the knight's chivalry and Ratbag's cowardice would create great tension and comedic moments. Another could be a magical wizard who constantly tries to force Ratbag to face his fears through various spells and challenges. Also, a sly thief from another world could team up with Ratbag, and together they get into all sorts of misadventures while the thief tries to teach Ratbag some of his more devious skills.
Definitely Luke Skywalker. He could bring his Jedi wisdom to Earth and teach people about the Force. Han Solo would also be great. His roguish charm would be fun to see on Earth. And of course, Darth Vader. His imposing presence would create a lot of drama.
Han Solo and a Skaven from Warhammer. Han's roguish charm and ability to get out of tough situations could be pitted against the devious and sneaky Skaven. The Skaven's underhanded tactics and Han's quick thinking would make for some really entertaining encounters as Han tries to outwit the Skaven in various situations across the two universes.
One character that would be really cool to include is Edward Elric from Fullmetal Alchemist. His knowledge of alchemy and his quest for the Philosopher's Stone could cross paths with Naruto's search for power to protect his village. Also, Saitama from One - Punch Man. It would be hilarious to see Saitama's nonchalant attitude towards fighting in the world of Naruto, where battles are usually more intense and drawn - out.
Definitely Darth Vader. His presence would create a lot of tension. He could be on a mission to hunt down the Rebels crew, and it would be a battle of wits and strength as they try to outmaneuver him.
Another plot idea is a crossover with a time - travel themed story. The characters from Summer Wars are sent to different time periods, like ancient Rome or the future. They have to adapt to the new cultures and technologies while also trying to find a way back home. Along the way, they might change historical events or gain knowledge that will help them save their own world from a potential threat.
Definitely Orochimaru. He is always after power and knowledge. In a gamer crossover, he would be the one trying to hack the game system or find some loopholes to gain an unfair advantage. And Hinata. Her shy but strong - willed nature could be really interesting in a game setting. Maybe she discovers some hidden powers within herself through the game mechanics that she was too shy to explore in the normal Naruto world.
Lara Croft from the 'Tomb Raider' series would be a great character to include. She's an expert adventurer and archaeologist. She could help the Men in Black uncover ancient alien artifacts or secrets that are related to current alien threats. Also, Hermione Granger from the 'Harry Potter' series. Her intelligence and knowledge of magic could be applied in unexpected ways in the world of the Men in Black. For example, she could use spells to help in situations where the Men in Black's technology fails.
One interesting character could be a powerful mage from the world Yuri crosses into. This mage could initially be wary of Yuri but then come to respect his skills. Another could be a godlike being who is a trickster. This character would constantly test Yuri and create humorous and challenging situations for him. And also, a young apprentice in this new world who looks up to Yuri and wants to learn from him.