Well, a cool fanfiction concept could be that some of the Resident Evil villains get transported to the Naruto universe. For example, Albert Wesker shows up and tries to take over the ninja villages using his superhuman abilities and intelligence. Or perhaps the Resident Evil bio - weapons are let loose in the Naruto world and the shinobi have to figure out how to deal with these new threats that are completely different from their usual enemies like other ninja clans or rogue ninjas.
One idea could be exploring the backstory of the Baker family in more detail. Maybe how they got infected in the first place and what their lives were like before the events of the game.
One idea could be exploring the backstory of the secondary characters more in - depth. For example, what was the life of the police officers in Raccoon City before the outbreak? Another could be a different take on how the main characters like Leon and Claire met and teamed up. Maybe they initially had some misunderstandings before realizing they needed to work together to survive.
One idea could be that Naruto and his team are sent to the Resident Evil universe to find a cure for a virus that has started to infect the Hidden Leaf. They would have to face off against zombies and bio - weapons using their ninja skills. Another plot could involve a Resident Evil character getting transported to the Naruto world and having to adapt to chakra and ninja society. For example, Leon Kennedy could team up with Naruto to stop a bio - terrorist attack in Konoha.
A possible plot is that some of the characters from Naruto are experimented on by the Umbrella Corporation. Sasuke, for instance, could have his Sharingan enhanced in unexpected ways due to the unethical experiments. This would lead to a power struggle within him as he tries to control his new abilities while also dealing with the fact that he has been turned into a sort of bio - weapon. And then Naruto would have to find a way to save him without getting infected himself.
A cool fanfiction plot might be to introduce a new character who has a special ability to communicate with the infected. This character could either help Leon or create new challenges for him as he tries to rescue Ashley.
Well, you could have an OC that is a journalist who gets caught up in the Resident Evil chaos while investigating a story. They then use their skills to document what's really going on, maybe even finding evidence that could stop the spread of the virus. Or perhaps an OC who was experimented on as a child by an unknown group related to Umbrella, and now has superhuman abilities that they use to fight back. Also, an OC that is a hacker trying to break into the systems of the bio - weapon creators to disrupt their plans.
One idea could be exploring the backstory of the villagers in more detail. Maybe they were normal people before being infected, and their lives and relationships could be fleshed out. Another idea is to rewrite a particular section of the game from a different character's perspective, like how Luis saw certain events unfold.
One idea could be creating an original character who is a scientist that was once part of the same research team as Simmons but had a change of heart. This oc could be on a mission to expose the real motives behind the C - virus outbreak. Another idea is an oc who is a skilled martial artist that teams up with Leon and Helena to fight off the infected in a city. They could bring a unique fighting style to the story.
You could also have an oc who is part of a secret organization that has been observing the events in Raccoon City all along. Their mission might be to contain the outbreak, but their motives are not entirely pure, adding a layer of mystery to the fanfiction.
Another idea is a story where the werewolves from Twilight are used in the Resident Evil world to fight the bio - organic weapons. The werewolves' natural abilities could give them an edge against the infected creatures, but they also have to deal with the sinister forces behind the Resident Evil outbreaks.