Maybe the gamer element could introduce quests into the RWBY universe. For example, the main characters are suddenly given a quest to find a rare relic that has been hidden in a digital - like dungeon. They have to use both their RWBY skills and the gamer - like knowledge of the character from the crossover to navigate and succeed in this new kind of adventure.
One idea could be that the gamer character brings in unique gaming skills like leveling up abilities faster than normal in the RWBY world. This could disrupt the balance of power among the teams and create new rivalries or alliances as others try to figure out this new 'gamer' ability.
Another plot idea could be that the gamer system malfunctions and starts creating glitches in the RWBY world. This could result in strange creatures emerging or areas of the world getting distorted. The main character then has to figure out how to fix the system while also dealing with the chaos it has caused.
One interesting plot idea could be the reincarnated gamer having unique abilities that they use to solve problems in the RWBY world in unexpected ways. For example, they might use their gaming knowledge of strategy to outwit Grimm.
One idea could be a character in the RWBY world suddenly getting a gamer system that allows them to level up their combat skills like in a video game. For example, they could see their 'HP' (health points) and 'MP' (mana points) bars, and earn experience by fighting Grimm. Another plot could involve using the gamer system to unlock hidden abilities or weapons within the RWBY universe. Maybe a character discovers a special 'quest' within the system that leads them to a powerful relic.
Perhaps the OC's Gamer system is glitched in a way that makes them perceive certain Grimm as video game bosses with specific attack patterns. They have to figure out these patterns to defeat the Grimm more effectively, adding an interesting twist to the battles in RWBY.
One interesting plot could be the characters from RWBY getting sucked into a virtual gaming world. They'd have to use their combat skills in new and unexpected ways, like dealing with game glitches as real threats. Maybe they'd encounter new digital enemies that are based on real - world fears but with a digital twist.
One idea could be that the Gamer's ability allows them to level up their Soul Reaper powers in Bleach. For example, they could start as a weak substitute Soul Reaper and through the Gamer system, they learn skills faster, like Ichigo did, but in a more gamified way. They might see stats for their Reiatsu, sword skills, and even their Hollow powers if they have any. Another plot could involve the Gamer character getting involved in the Soul Society's politics. Since they view everything like a game, they might accidentally or intentionally disrupt the traditional power balance while trying to complete quests related to the Soul Society.
One idea could be that Naruto gets a gamer power that allows him to level up his ninja skills like in a game. For example, he can see his progress in learning new jutsus as leveling up. Another could be a crossover with a popular game world where Naruto has to fight game bosses using his ninja abilities.
The gamer OC could be from our world and is somehow sucked into the RWBY world. In this new world, he realizes that his knowledge of games gives him an edge. He starts to form a harem as he helps the RWBY characters in different ways, like providing strategic advice during battles. For instance, he might suggest new ways to fight the Grimm based on his gaming experience. The relationships in the harem could develop as they train together and the OC shows his determination and kindness. Some characters might be initially skeptical of him but gradually warm up due to his positive influence.
A possible plot is that a portal opens between the Doom universe and RWBY's. Some of the Doom demons start spilling into Remnant, the world of RWBY. Ruby and her team initially mistake them for a new, extremely powerful type of Grimm. As they try to fight them, they realize they need different strategies. Meanwhile, the Doom Slayer follows the demons through the portal to stop them. Eventually, the two groups meet and learn from each other's combat styles to defeat the invading demons.
Well, a cool plot for a Witcher RWBY crossover fanfiction might involve the Witcher's magic and potions being introduced to the RWBY universe. For example, a potion that gives the RWBY characters temporary enhanced senses like a Witcher. Or perhaps a situation where a powerful Witcher villain comes to the RWBY world and the teams have to combine their skills to defeat him. The different fighting styles of the Witcher and RWBY characters would make for some really exciting battle scenes.