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.
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 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.
Well, Ron might be initially terrified of Slenderman. But when his friends are in danger, he would step up. The Weasley twins could try to play pranks on Slenderman, not fully realizing the danger at first. Dumbledore, on the other hand, would be very cautious. He might observe Slenderman from a distance, trying to understand his nature before taking any action. In a 'Harry Potter and Slenderman fanfic', these interactions would create a lot of interesting scenarios.
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.
One idea could be that the Slenderman somehow enters the RWBY universe during a Grimm attack. The characters of RWBY initially mistake him for a new type of Grimm. But as they start to investigate, they realize he has very different and more sinister powers compared to the Grimm.
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.
Weiss, with her more refined upbringing, could initially be a bit shocked by the differences in the Yamato's world but would soon start to find ways to use her skills. She might help with strategic planning, using her knowledge from Beacon Academy.
The characters would likely have an initial period of confusion. The Warframe Tenno, being used to their own high - tech battles, might be startled by the more 'primitive' but still effective weapons of RWBY. Meanwhile, the RWBY characters could be in awe of the Warframes' advanced abilities. For example, when a Warframe uses teleportation, the RWBY team would be both amazed and a bit suspicious. Eventually, they would start to work together, with the Warframes teaching the RWBY characters about advanced combat tactics and the RWBY characters showing the Warframes the ins and outs of their world.
Ruby would probably be very excited to meet Geralt. She might ask him a million questions about his adventures and his weapons. Geralt, on the other hand, would be more reserved but would respect Ruby's enthusiasm. Blake might be a bit cautious around the Witcher at first, but would soon be interested in his knowledge of different creatures. Yang would be eager to spar with Geralt to test her strength. And Weiss would be curious about the Witcher's magic and how it compares to her own Glyphs.