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.
Imagine if the Grimm started to invade other universes. Ben, being a hero in his own right, would sense the danger and travel to Remnant. There, he meets the RWBY characters. Ruby would be fascinated by Ben's Omnitrix and its ability to transform him into different aliens. Ben, on the other hand, would be intrigued by the Huntsmen and Huntresses' combat skills and the Dust - based technology. They would then join forces to stop the Grimm invasion from spreading further.
Well, from what we can gather, it seems the RWBY characters will be important. Maybe Ruby, Weiss, Blake, and Yang could be involved as they are the main characters in RWBY. And then there's this 'Ben' character. It's possible that the story focuses on their interactions, especially with the RWBY characters being in an attacking role towards Ben. However, it could also introduce other secondary characters from the RWBY universe or even create new ones for the fanfiction's plot.
Blake could use her stealth and agility to observe the Slenderman from a distance. She might notice patterns in his behavior that the others don't. This would then be used to help the team come up with a more effective plan to either avoid or confront the Slenderman.
Well, without having read it directly, it could potentially be about RWBY characters attacking this 'Ben' in a scenario that the author of the fanfiction has devised. Maybe there's some sort of power struggle or personal vendetta within the RWBY universe combined with the addition of this 'Ben' character. It could also be a misinterpretation of a more complex relationship that the fanfiction is exploring.
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.
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.
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.
The gamer could interact with Ruby by showing her the unique weapons that can be created with UBW. Ruby, being a weapon enthusiast, would be extremely interested. They might also team up during battles, with the gamer using UBW to support Ruby's scythe - based combat style.
RWBY characters might initially be confused by Earthlings' lack of knowledge about Dust and Semblances. They could start by trying to educate some curious Earthlings about their world. For example, Ruby might show off her Crescent Rose to a group of engineering students, who are fascinated by its design.
The Stargate Atlantis characters are used to dealing with advanced technology and alien species. They might initially be very curious about the RWBY characters' abilities that seem almost like magic in comparison. For example, Sheppard might be intrigued by Yang's ability to absorb and release energy with her punches.
The RWBY characters could interact in multiple ways. Ruby would be the one to inspire hope in the Doom - like situations in the fanfiction. Weiss would use her strategic thinking to deal with the challenges. Blake, with her ability to adapt, would find ways to survive. Yang, being the powerhouse, would directly engage with the threats, much like she does in her own world against Grimm. They would all bring their unique Remnant - based skills and personalities to interact with the Doom concepts.