One idea could be creating an OC who is a former NCR soldier turned bounty hunter. They could have a complex backstory of disillusionment with the NCR's politics and find a new purpose in tracking down dangerous criminals in the Mojave Wasteland.
An interesting OC could be a ghoul scientist. They were once a human scientist before the Great War, and now, as a ghoul, they continue their research in a hidden bunker. Their fanfiction could revolve around their struggle to be accepted by the other inhabitants of the Mojave, while also trying to make scientific breakthroughs that could change the wasteland.
One idea could be creating an OC who is a hunter from RWBY world but ends up in the Fallout New Vegas setting. They could use their unique weapon skills from RWBY, like using a scythe in combat, and adapt them to the post - apocalyptic world. Maybe they team up with some of the factions in New Vegas while also trying to find a way back home.
A plot idea could be that the Courier discovers a powerful relic in the Fallout world that has the power to connect different universes. This relic accidentally transports the Courier and some of the key RWBY characters to a strange in - between realm. In this realm, they have to work together to find a way back to their respective worlds while also facing challenges that are a mix of both Fallout and RWBY elements, like Grimm - like creatures with post - apocalyptic mutations and bandits with Dust - powered weapons.
A plot could involve a group of settlers from Fallout 3 trying to make a new start in the Mojave. They run into trouble with the various factions in New Vegas. Meanwhile, a character from New Vegas goes to the Capital Wasteland to learn about their different ways of survival. This leads to an exchange of ideas and technology between the two groups, as well as some clashes due to different values.
You could write a fanfic about the origin of one of the factions in Fallout New Vegas. Let's say the Powder Gangers. Maybe they were once just a group of miners who were mistreated and decided to take up arms. Show how their ideology evolved from a simple quest for justice to the more chaotic and violent group we see in the game.
One plot idea could be a team of RWBY characters getting teleported into the Fallout New Vegas wasteland. They'd have to adapt to the harsh environment, deal with radiation, and face off against the various factions in the game. Another could be a character from Fallout New Vegas traveling to the RWBY world and learning about the concept of Dust and Huntsmen.
A more complex plot might be that there is a dimensional rift that is causing chaos in both universes. Characters from both Fallout New Vegas and RWBY have to work together to close the rift. In the process, they learn about each other's worlds, values, and fighting styles. The contrast between the post - apocalyptic technology and magic - based abilities in RWBY could lead to some really interesting interactions and story developments.
One idea could be exploring the backstory of a minor character in the game, like a random caravan trader. You could create a whole story about how they got into the caravan business, their adventures on the roads, and the dangerous encounters they've had. Another plot could revolve around a new faction emerging in the Mojave Wasteland, and how it affects the existing power balance among the NCR, Legion, and Mr. House. Maybe this new faction has a unique ideology or technology that shakes things up.
One idea could be an OC who is a brilliant scientist in the Fallout world. They could be working on a secret project to reverse the effects of radiation. Another idea is an OC who is a wandering trader, having all kinds of adventures while traveling from one settlement to another, facing raiders and mutants along the way. And a third idea could be an OC that is part of a new faction trying to bring peace and order to the wasteland.
One interesting story is about the Courier's journey to find out who shot them. It's a quest for revenge and truth that leads through many dangerous areas and encounters with various factions in the Mojave Wasteland.
There are fanfictions that focus on the relationships between the different factions in the game. A story could be about a character trying to broker peace between the NCR and the Legion. This would involve a lot of political intrigue and difficult decisions as the character has to deal with the different ideologies of each faction.