Another way is through a conflict that forces them to work together. Say an evil force from Ben's world wants to steal the magic from Equestria Girls' world. Ben and the girls have to put aside their differences and combine their powers. Ben might use his XLR8 form to quickly move around and gather information, while the girls use their magic to set up defenses or attacks. Their interaction would be filled with moments of discovery as they figure out how to best use each other's skills.
The characters can interact through shared adventures. For instance, Ben and the Equestria Girls could go on a quest to find a magical item that has been lost. Ben's abilities with his alien forms can help in combat or traversing difficult terrains, while the girls' magic and knowledge of their world can be useful in solving puzzles related to the item.
They could also interact in a more social setting. Ben could attend Canterlot High for a while. He might befriend some of the girls, like Sunset Shimmer. They could exchange stories about their different worlds. Ben could tell them about his battles against villains in his universe, and the girls could share their experiences with magic and friendship. This would create a really interesting dynamic as they learn from each other.
For the characters to interact realistically in this fanfiction, their goals and motivations need to be taken into account. Harry may be motivated by the need to find his way back home, while the Equestria Girls might be focused on maintaining peace in their school or town. They can come together when they realize that their individual goals can be achieved by working together. Maybe Harry's knowledge of magic can help the girls protect their world from a new threat, and in return, they can use their resources to help him find a way back to his own world.
Another way of interaction is through a shared mission. Let's say there is a threat that is causing chaos in both Ben's world and the JLU's world. They have to work together to stop it. Ben might use his Heatblast form to deal with some fire - related aspects of the threat, while Superman could use his strength to move large objects. During this mission, they communicate, strategize, and support each other, which leads to a deeper understanding and friendship between the two groups of characters.
In fanfiction, they might interact due to a misunderstanding at first. Maybe the Justice League sees Ben using his alien powers and thinks he's a threat. But then they learn that there are other forces at work that are endangering both their worlds. So they start to communicate and plan together. Ben can show the League his knowledge of different alien races and technologies, while the League can share their experience in dealing with big - scale threats on Earth.
Ben would probably be the protector and the guide in some ways. He'd use his knowledge of the Omnitrix and his experiences with different aliens to help the monster girls. For example, if a monster girl is scared of a certain type of alien, Ben can transform into that alien in a non - threatening way to show her there's nothing to fear.
The characters could interact through a portal that connects their two universes. For instance, Ben could be exploring different dimensions and stumble upon Remnant. Then he meets the RWBY team, and they start to exchange knowledge about their respective worlds.
The human character usually starts off as an outsider. The ponies are curious about this strange new being. Some ponies might be friendly right away, like Pinkie Pie who would probably throw a welcome party. Others might be more cautious, like Twilight Sparkle, who would want to study the human first.
A cool fanfiction concept could be a crossover tournament. Ben 10 brings some of his alien allies, and the Equestria Girls bring their friends from Canterlot High. They compete in a series of challenges that test both strength and magic. Each team has to learn from the other's abilities and work together at times. This could lead to some interesting friendships and rivalries between the characters from the two different universes.
The Autobots could see Ben as an interesting new ally. They might be initially cautious, but as they see him use his Omnitrix to transform and fight, they gain respect for his abilities. Ben, on the other hand, would be in awe of the Transformers' size and power. He could learn from them about advanced technology and use that knowledge to upgrade his Omnitrix. The Decepticons would likely see Ben as a threat or a tool to be used. They might try to capture him or the Omnitrix. Ben's alien forms could have different interactions with the Transformers. For example, XLR8 could be used to quickly scout out Decepticon bases, while Diamondhead could provide a strong defense against their attacks.
Some of Ben's more heroic aliens, like Upchuck, could have really funny interactions. Upchuck might accidentally eat some of the devil fruits or other magical items in the DxD world, causing all sorts of wacky effects. And the angels in DxD might be initially suspicious of Ben, thinking he's some kind of new unknown entity. But as they see him fight against common enemies, they could start to respect him. Ben, on the other hand, would find the whole concept of the different supernatural races in DxD fascinating and might want to learn more about their powers.
Ben might initially be very cautious around the Xenomorphs. He's used to his own set of aliens, but these are something entirely different. He could try to communicate with them using his alien forms' unique abilities, but it might not work as the Xenomorphs are more about instinct. The Xenomorphs, on the other hand, would probably see Ben as a threat or a potential host.