One possible plot is that the Family Guy characters visit Remnant. Peter could try to eat all the strange foods in Remnant, causing chaos. Lois might befriend the female characters of RWBY and they could exchange stories about their different worlds. Chris could be in awe of the Huntsmen and try to learn some of their skills.
Well, it could be about the RWBY characters somehow ending up in Quahog. Ruby might befriend Stewie and they could go on some wacky adventures together. Maybe Weiss could have a culture - shock moment when she meets the Griffin family. And Blake could get into some interesting discussions with Brian about literature or something.
Perhaps it would involve the characters from Family Guy somehow getting trapped in the Underground of Undertale. Peter Griffin could try to befriend the monsters there, like Toriel, while Stewie might be intrigued by the unique magic system and try to use it for his own evil (or not so evil) plans. The different art styles and tones of the two shows would create an interesting contrast in the fanfic.
In a 'rwby x special forces fanfic', it could be that the special forces are from another dimension. They accidentally end up in Remnant and meet the RWBY teams. At first, there's mistrust, but as they realize they have a common goal of protecting the innocent, they start to collaborate. There could be training sequences where the RWBY characters teach the special forces about Aura and the special forces teach them about modern combat tactics.
One possible plot could be that Naruto somehow gets transported to the RWBY universe. He teams up with Neo. Maybe they start off as rivals, with Neo being distrustful of Naruto at first because of his strange powers. But as they face common enemies, like the Grimm in the RWBY world, they learn to work together and combine their unique abilities, like Naruto's jutsu and Neo's semblance.
One interesting crossover could be the Griffins finding themselves in the world of RWBY. Maybe Stewie would be intrigued by the unique weapons and semblances. He might try to figure out how to incorporate some of those elements into his own evil plans.
The plot could involve Chris's pregnancy being a mystery at first. The Griffins start to notice strange behavior from Chris, like sudden cravings and mood swings. As they discover he's pregnant, they have to navigate the social stigma in Quahog. Chris could struggle with his identity during the pregnancy, and the family has to come together to support him. There could also be some sort of supernatural or comical explanation for how he became pregnant, like a wacky experiment gone wrong or a magical mishap.
Blake and Brian could be an amazing duo. Blake, with her love for books and her complex past, and Brian, the intelligent dog who is also a bit of a philosopher. They could have deep conversations about ethics and morality, all while being in a world that is a mash - up of RWBY and Family Guy.
Well, perhaps the hot dog guy is a new arrival in Gumball's town. Gumball, being the curious cat he is, befriends the hot dog guy. Their story could revolve around the hot dog guy introducing Gumball to new street food concepts and Gumball showing the hot dog guy the wacky and wonderful aspects of his school and family life. As they spend more time together, they form a strong bond that helps them both grow as characters.
The plot could start with the girl finding the scared guy in a lemon grove. Maybe he's lost and she offers to help him. As they search for the way out, she gives him some lemons to snack on. The taste of the lemons makes him relax a bit. They start to share their stories, and the girl's positive energy slowly chases away his fear.
Well, in a 'rwby meet the family fanfic', it could center around the main RWBY characters bringing their friends to meet their families. For example, Ruby might take her team to her home in Patch. There could be interesting scenarios where they learn about each other's family traditions. Maybe Weiss's family's strictness could be contrasted with Yang's more laid - back family environment. It could also be a story about the characters' families coming together to support them during a difficult time in their Huntsman/Huntress training.
Well, it could be about a giantess character in the Family Guy universe causing chaos. Maybe she's a new neighbor who is so large that she accidentally steps on cars or buildings, and the Griffin family has to figure out how to deal with her. It could also involve some comical misunderstandings where her size makes normal situations go haywire.