A plotline could involve Si's genderbent self having to prove themselves in a world that may have different perceptions of their new gender. They could struggle with how others view them and at the same time, discover new aspects of their own abilities. Maybe they were underestimated in their original form, but as a genderbent character, they show everyone what they're truly capable of. It could also involve a romantic sub - plot where their new gender changes the dynamics of their relationships with other characters in the RWBY universe.
I'm not sure of specific 'Si gets genderbent RWBY fanfiction' titles off the top of my head. You could try searching on fanfiction platforms like Archive of Our Own. There are often filters you can use to find fanfics related to genderbending in the RWBY universe.
A possible plotline could be that a new enemy emerges in the RWBY world, a puppet master who starts to take over the minds of the hunters and huntresses. The RWBY team has to figure out how to stop this enemy before everyone they know becomes a puppet under the enemy's control.
Perhaps there is a common enemy that forces Captain America and the RWBY characters to work together. This enemy could be a powerful being that has the ability to manipulate both the technology in Captain America's world and the Dust in RWBY's world. The combined forces of the two universes would have to find a way to defeat this enemy and in the process, learn from each other's strengths and weaknesses.
One possible plotline could be that it's the result of some strange magic or alien technology. Maybe an enemy uses a device on Goku as part of a plan to weaken him or create chaos. Another could be a more comical take where Goku accidentally ingests something that causes this unusual situation.
Maybe in the fanfiction, Ash is given a Chespin by a mysterious trainer. This Chespin has some special powers that Ash has to figure out how to unlock. As they train together, they form a deep bond and go on to compete in a big Pokémon tournament where Chespin's unique abilities give them an edge.
Another plotline might involve Naruto being ostracized because of his new Mokuton power. Since it was associated with the Uchiha and Senju in a complex way, some in the ninja world might suspect him of having ulterior motives. This would lead to Naruto having to prove himself all over again, this time not just as a jinchuriki but also as a wielder of Mokuton.
A plotline might involve Ash being betrayed by a new acquaintance he meets on his journey. This person could pretend to be his friend, gain his trust, and then use that trust to steal his Pokémon or important items. Or perhaps Ash is betrayed by a Pokémon he just caught. The Pokémon could have been planted by an enemy trainer to spy on Ash and then turn on him at a crucial moment.
In some 'ginny gets drunk fanfiction', the plot might revolve around Ginny being in a new and unfamiliar environment where she gets drunk. This could lead to her exploring a different side of herself. She might meet new people and form new relationships while in this state. The story could then be about how this one drunken night changes her perspective on certain things or people in her life. It could also show her growth as a character as she learns from the experience of getting drunk and the events that follow.
Maybe a plot where Glynda's telepathy is somehow affected by an outside force. This could lead to her losing control over it at times and causing chaos, and she has to find a way to regain control and fix the problems it causes. It could also involve her learning more about her own power through this struggle.
One possible plotline could be that Ruby is kidnapped by a powerful new enemy. This enemy might be after her silver eyes power and plans to use it for some evil scheme. Another could be that Ruby accidentally gets transported to another dimension during a battle, leaving her team to search for her.