Sure, there are fanfictions like that. Some of them focus on the cultural differences between the two groups. The Avengers, coming from a more modern Earth - like setting, and the RWBY characters from their own unique world. How they communicate, misunderstand each other at first, and then gradually come to understand and work together is often a big part of these fanfictions.
The OC might interact with them in various ways. They could start by being in awe of their combat skills, like Ruby's speed. And in the harem aspect, they could show affection differently to each character. For Weiss, it could be through respecting her family name and helping her with her studies. With Blake, they might bond over books and their love for knowledge.
The more serious Avengers like Natasha Romanoff and Clint Barton could have in - depth conversations with Blake. Blake's complex backstory and her views on the world might interest them. On the other hand, Thor and Yang could end up in a friendly sparring match. Thor would be impressed by Yang's strength and Yang would be excited to fight someone as powerful as Thor. The RWBY characters could also learn a lot from the Avengers' combat experience and strategic thinking.
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.
Ruby would likely try to find a way to end the war quickly. She has a strong sense of justice and wouldn't stand for all the death and destruction. Weiss might use her knowledge of politics and strategy to try and influence the leaders in the fanfiction. Blake would probably be more on the sidelines, observing and trying to understand the different sides of the conflict. Yang would be right in the middle of the action, trying to protect the innocent and fight against the unjust forces.
In some crossovers, Adam and RWBY characters interact in a confrontational way. For example, Adam may see the RWBY team as a threat to his own goals and try to eliminate them. This often leads to intense battles where the RWBY characters have to use their combined skills to fend off Adam's attacks.
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.
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.
They can fit in various ways. Ruby, with her leadership skills and powerful scythe - fighting abilities, could be a valuable asset in the fight against Thanos. Maybe she'd befriend Spider - Man and they'd team up to take on some of Thanos' minions. Yang's strength and determination would make her a great addition to the Avengers' lineup, perhaps taking on the role of a front - line fighter like Captain America.
Obviously, Ruby, Weiss, Blake, and Yang would be main characters. They are the core of RWBY. Ruby with her speed and scythe skills would be crucial in any fight. Weiss, with her ice powers, could freeze some of the terrifying enemies from 'The Evil Within'.