In the case of Lucy, Erza could be a good addition. Erza's strong leadership and combat skills could blend well with Lucy's strategic thinking. As for Hibiki, Genis would be an exciting partner. Genis' intelligence and Hibiki's physical abilities could create a great dynamic in the fanfiction.
Luna could also be a good match. Luna's laid - back and cool personality could contrast well with Lucy's more somber and mysterious vampire self. They could bond over their 'outsider' status in a way, with Luna being the rocker chick and Lucy being a vampire. Maybe they could form a band where Lucy uses her vampire - like charm to draw in the crowds.
Natsu could be a great character to pair them with. Natsu's fiery and impulsive nature would contrast well with Lucy's more level - headed and Erza's strict personality. They could all go on an adventure together, with Natsu leading the charge into dangerous situations, Lucy using her celestial spirits for support, and Erza keeping them in line. Gray is also a good option. His cool - headedness and ice - make magic could add an interesting dynamic to the group, especially when paired with Natsu's fire magic and the celestial and re - equip magic of Lucy and Erza.
Gray could be another great pairing. The contrast between his cool - headedness and Lucy's more emotional nature could create interesting storylines. For instance, they might be forced to work together on a mission where their differences initially cause problems but later lead to a stronger bond.
For Hibiki, his character could be developed by exploring his relationship with his guild more deeply in the context of his interactions with Lucy. Maybe he has some insecurities about not being as strong as some of the other guild members, and Lucy helps him see his own value through their adventures.
Chat Noir would be a great pairing. Their different powers and personalities could create some really fun and exciting interactions. Chat Noir's flirty nature combined with Multimouse's more cautious approach would make for an interesting dynamic.
Goku is a great character to pair with Beerus. Their relationship in the original series is full of tension and mutual respect. In fanfiction, they could have all sorts of adventures together, like exploring new galaxies or training in unique ways.
Chloe could be a great addition. She could be a supportive friend who helps Clana in their relationship, or there could be some friendly jealousy triangle situations that add drama. Another good character to pair with them could be Lex. His complex relationship with both Clark and Lana in the original show can be further explored in fanfiction, like how he might try to break them up or use their relationship for his own gain.
Catwoman could also be paired with Batgirl in fanfiction. They have an interesting dynamic as Catwoman is more of an anti - hero while Batgirl is a full - fledged hero. Their encounters could range from tense stand - offs to unexpected alliances as they both navigate the seedy underbelly of Gotham.
One idea could be that Lucy and Hibiki are from different worlds but are brought together by a magical accident. They then have to team up to find a way back to their respective homes while facing various challenges along the way.
You could also try looking for authors who are very active in the Naruto fanfiction community. They are more likely to be up - to - date with the latest trends and ideas in the fandom, and might be more inclined to write interesting stories about Hikari and Hibiki.