Well, perhaps Harry Potter, using his magic, accidentally creates the world of Remnant from RWBY. He might then have to deal with the new and strange creatures and powers there, while also trying to find a way to get back to his own world.
There may be. However, finding it might take some effort. You could also try joining Harry Potter or RWBY - focused fan groups on Facebook or Discord. In these groups, you can post a query asking if anyone has read or written a fanfiction where Harry Potter creates Remnant RWBY. Members are usually quite helpful and might be able to point you in the right direction.
One idea could be that Harry creates a school for magical creatures. He realizes that there are so many unique and often misunderstood creatures in the wizarding world, and he wants to give them a place to learn and grow. This school could teach them how to interact with wizards in a positive way, as well as develop their own magical abilities.
The'mother of remnant' in RWBY fanfiction is a highly variable concept. It could be a long - ago queen who united the early tribes and laid the foundation for the kingdoms, so she is seen as the mother in a metaphorical sense. Or it could be a being from another dimension who came to Remnant and started the cycle of life and death there. Fanfic writers have a lot of creative freedom with this concept, so there are countless possibilities for who she could be.
One idea could be that Harry discovers an ancient spell book in the Room of Requirement that has the incantations for creating a magic railgun. He might use it to defend Hogwarts against a new, powerful dark force that is immune to traditional magic spells.
In this fanfiction, Harry might also include some of the Hogwarts ghosts in his army. After all, they have a long - standing relationship with the school. Peeves, for example, could be a wild card in the army. He can cause chaos among the enemies. And some of the older students who have a strong sense of justice, like Cedric Diggory's friends, could also be part of it.
If Harry Potter goes to Remnant in fanfiction, it could be a wild adventure. He'd probably stand out a great deal. His wand - a very important tool in his world - might not work in the same way in Remnant. He could end up learning new forms of combat and magic there. He may also encounter different types of villains and have to use his quick thinking and skills to survive and help those in need.
The plot could start with Harry finding an ancient Jedi artifact during one of his adventures. This leads him to become curious about lightsabers. He then uses his magic knowledge to try and replicate it. Along the way, he might face opposition from some in the wizarding world who think it's too dangerous or against wizarding traditions. But Harry, being the hero he is, persists. Eventually, he creates a lightsaber and uses it to defeat a new, powerful dark wizard who has emerged, using both his wizarding skills and his new lightsaber skills.
In a fanfiction where Harry Potter creates a virus, perhaps it's a virus that affects magic itself. The virus could spread through spells and start to corrupt the very essence of magic. Harry, realizing the severity, might then have to team up with his old friends like Hermione and Ron. They would search for ancient magic or rare potions to counteract the virus before it destroys the entire magical ecosystem.
In a 'Harry Potter creates a shelter' fanfiction, perhaps Harry uses his magic to create a shelter in the Forbidden Forest. He might use spells like Protego Totalum to make it safe from dark creatures. It could be a place where he hides his friends during a dangerous time at Hogwarts, like when Voldemort's forces are on the prowl.