Gray could be a great addition. Since Gray has a history with both Juvia and Lyon, his presence can add a lot of drama. For instance, his reaction to Juvia and Lyon getting closer could range from jealousy to acceptance, which would create different story arcs.
One interesting plot could be Juvia and Lyon going on a magical adventure together to find a lost artifact. They might face various challenges along the way that test their skills and relationship.
For Juvia, she could be developed by exploring her insecurities more deeply. Maybe she has a fear of losing Gray not just to another woman, but also to his own self - destructive tendencies. She could learn to be more independent while still maintaining her love for Gray. As for Lyon, he could be developed by showing his growth beyond his rivalry with Gray. He could start to focus more on his own identity and magic, and how he can contribute to the Fairy Tail guild in a unique way. Gray can be developed by confronting his past traumas more directly. He might have nightmares about his master's death, and through the support of Juvia and Lyon, he learns to heal and become a stronger wizard.
One idea could be a story where Juvia, Lyon, and Gray have to team up to face a powerful dark wizard that is threatening the Fairy Tail guild. They start off with some tension due to their past relationships, especially between Gray and Lyon who have a rivalry, but as they battle, they learn to work together better. Juvia could also be in a position where she has to choose between protecting Gray or following Lyon's more strategic plans, adding an interesting love - triangle - like dynamic to the plot.
In a 'Juvia Gets Drunk' fanfiction, Juvia would most likely interact with the members of Fairy Tail. She might start bothering Natsu, asking him silly questions like if he can really breathe fire while drunk. She could also interact with Happy, and they might get into some sort of funny exchange, like Juvia trying to teach Happy a new song she made up in her drunken state. And of course, she'll interact with Cana. Cana being the drinker she is, might find Juvia's drunk behavior either hilarious or relatable and they could have a long chat about different kinds of drinks.
Their relationship would likely become more serious. They would have to think about not just themselves but also their baby. There would be more commitment and responsibility between them.