Perhaps the new thief has a hidden agenda. They might be stealing not just for personal gain but to expose some dark secrets in the RWBY universe. For example, they could be stealing information from the villains that could help the heroes in the long run. The story could then revolve around the thief's interactions with the various characters in RWBY, both good and bad, as they try to achieve their goals while avoiding getting caught.
It could be about a new character who is a thief and gets involved in the world of RWBY. Maybe they steal an important relic and the RWBY team has to track them down.
The main character would most likely be the new thief. They could be an original character created just for this fanfiction.
One possible plot is that Harry becomes a thief out of desperation. His friends are in danger, and the only way he can get the resources to save them is by stealing from the rich and powerful in the wizarding world. This leads him on a journey of self - discovery, as he struggles with his new identity and the moral implications of his actions. As he steals, he also discovers a hidden underworld in the wizarding community that he never knew existed, filled with its own rules and power dynamics.
Well, one plot twist could be that Jaune discovers a hidden power within himself upon transferring. Maybe he finds out he has a special connection to an ancient relic at the new school.
One possible plot could be that the characters face a new, more powerful type of Grimm that has abilities they've never seen before. They have to quickly adapt and find new ways to fight. Maybe Ruby discovers a new form of her scythe's power to combat this threat.
A plot could revolve around a new tournament in the RWBY universe specifically for the new game. Characters from different academies form teams not only to compete but also to uncover a conspiracy that seems to be using the tournament as a cover. It would involve a lot of intense battles, strategic planning, and some mystery-solving.
A really interesting plot for this fanfiction could be that the Avengers are somehow drained of their powers and they need to find a way to get them back. They hear about the power of the Greek gods from 'The Lightning Thief'. So they travel to that world and meet Percy. Percy, being the hero he is, decides to help them. They then go on a journey through the different realms of the Greek gods, facing monsters and challenges together. Along the way, the Avengers learn about the importance of teamwork in Percy's world and Percy learns some new combat and strategic skills from the Avengers.
It could be about a duelist who secretly steals rare cards at night. Maybe he's a desperate collector or someone being forced to do it by a more powerful figure in the Yugioh underworld.
Maybe the plot could revolve around Abu trying to escape hell. Since he was a thief in life, he might use his cunning skills to find a loophole or a way out of the eternal damnation.
New weapons can introduce new challenges. For example, if a character has a weapon that can manipulate time, it can create complex plot twists. Enemies might try to steal it, or the character might accidentally cause chaos with it.
One possible plot could be Trump arriving in the RWBY world through some sort of magical portal. He might initially be confused but then start to apply his business - like mindset to the problems in the RWBY world, like trying to negotiate with the Grimm. Another could be that he becomes a mentor - like figure to one of the RWBY characters, teaching them about leadership in his own unique way.