How about the idea of the ninjas from Naruto trying to find a way to reverse the radiation in Fallout 4? They could use their chakra knowledge and skills. Hinata could use her byakugan to detect sources of radiation more accurately. And Naruto could use his positive energy and willpower, which is often associated with his chakra, to try and purify some of the polluted areas.
One idea could be that the characters from RWBY end up in the Fallout wasteland. Ruby could use her scythe skills to fight off raiders and mutants. Weiss might use her glyphs to protect the group from radiation in certain areas. Another idea is that the Dust from RWBY could be a valuable resource in the Fallout world, leading to various power struggles.
One idea could be a story about a lone wanderer who discovers a hidden vault that was never part of the original vault - Tec plan. They find it filled with advanced technology that could change the wasteland. Another could be a fanfiction where a group of settlers tries to build a new civilization based on pre - war values in the middle of the irradiated chaos. And also, a story about a mutant who has the ability to communicate with the mutated creatures and tries to broker peace between them and the humans.
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.
Another interesting plot could involve a female deathclaw being an outcast among her own kind. In the fanfiction, she could be more intelligent and empathetic than the other deathclaws. She decides to seek out the source of some strange occurrences in the wasteland that are affecting both humans and deathclaws. Along the way, she teams up with a group of survivors who are initially afraid of her but soon learn to trust her.
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.
One plot idea could be a story where a lone wanderer befriends a deathclaw in a post - apocalyptic world filled with the remnants of Fallout. Maybe the deathclaw has some sort of unique mutation that makes it more intelligent and less aggressive. Another could be a group of survivors trying to study deathclaws and accidentally form a bond with one through a series of comical and dangerous encounters. A third idea might be a story from the perspective of a deathclaw itself, showing its daily life in the wasteland and how it views the humans.
A plot could involve Cass's pregnancy being a symbol of hope in the Fallout wasteland. She might decide to start a community initiative for better living conditions for the upcoming baby. For example, she could try to clean up and secure an area for a future settlement. This would also show her determination and strength during pregnancy.
One idea could be that the RWBY characters find themselves in the Fallout wasteland. Ruby could use her Crescent Rose to fend off mutants. Weiss might struggle with the harsh and dirty environment compared to her usual clean Atlesian surroundings. Blake could use her stealth skills to navigate dangerous areas, and Yang's strength would be useful in combat against raiders. They could be on a quest to find a way back to their own world while also trying to make the wasteland a better place.
One idea could be to cross the Sole Survivor with a character from another post - apocalyptic game like Metro. The Sole Survivor could bring their unique skills and knowledge from the Fallout universe into the Metro tunnels. They might have to deal with different kinds of mutants and a more claustrophobic environment. It would be interesting to see how they adapt and interact with the Metro characters.