A new villain could be introduced. This villain might be related to an old enemy but with a new and more terrifying power. Ruby has to figure out how to defeat this villain while also protecting her friends.
Perhaps a mysterious mentor figure appears. This person has seen the rise and fall of many kingdoms and has come to guide Ruby in her fight against the darkness.
Emerald might be a character in this fanfiction. She could start as an enemy, trying to steal the Fall Maiden power from Ruby for Cinder. But as the story progresses, she might see the good in Ruby and switch sides. Neptune could also be there, providing some comic relief in the midst of all the serious Maiden - related business. And Glynda could be involved in training Ruby to control her new powers.
It could be that Ruby has to deal with the consequences of a major decision she made in the past. This causes her to question her leadership skills and she embarks on a journey of self - discovery to become a better leader for her team.
Another type of new character could be a mysterious figure who appears in Naruto's future vision. This character might have powers that are beyond comprehension at first. They could be a key to unlocking a new form of chakra or a secret about the origin of the ninja world. Their motives would be unclear at first, adding an element of mystery to the fanfiction.
One plotline could be Ruby's death leading to a power vacuum. Since she's a powerful and important figure in the RWBY world, her death could cause chaos. Her friends might struggle to fill the void she leaves, both in terms of leadership and combat ability. For instance, they might have to quickly learn to work without her Crescent Rose and her strategic mind.
Well, her sister Yang would definitely be one of the main characters. Yang would be very protective of Ruby and would try to help her in any way she could. Weiss would also be important. She might use her intelligence and Dust - related skills to assist Ruby. And Blake, with her stealth and combat skills, could be crucial in protecting Ruby and guiding her through difficult situations.
There could be a new tech - savvy kid who idolizes Hiro and wants to join the team. This character might bring in fresh ideas and a different perspective on using technology for good.
Blake is also a common character to be paired with Ruby. There could be fanfictions where they team up for a special mission that is different from the main storyline. For example, they might have to infiltrate a secret organization together, relying on each other's unique skills.
One character could be Nora. Nora is energetic and fun - loving, just like Ruby in some ways. Their combined energy could make for a very lively threesome scenario in fanfiction. Ren might also be a candidate. Ren's calm and collected nature would contrast well with Ruby's more exuberant personality. Together with another character, perhaps Weiss, they could form an interesting threesome. Also, Emerald could be in a threesome with Ruby. Emerald has her own motives and a somewhat complex personality. Her interactions with Ruby, especially in a threesome situation, could add a lot of depth to the fanfiction.
In some cases, new original characters might be introduced. These could be other hunters or huntresses who have a connection to the source of Ruby's darkness. They could either be helping Ruby or be part of the problem. Maybe they are also victims of the same dark power that is corrupting Ruby and they both have to find a way out together.
In the fanfiction, Ruby as a demon may have the power of flight. Many demons in fictional works have wings or the ability to levitate. This would give her an edge in battles, allowing her to quickly move around the battlefield. Additionally, she could have the power of regeneration. Demons are sometimes shown as being able to heal from wounds quickly, and this new Ruby might be able to do the same, making her a more formidable opponent.