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.
Erza could also be included. She is a strong and authoritative figure. In the fanfiction, she might be the one who assigns Juvia and Lyon on a mission together. Her no - nonsense attitude could influence how Juvia and Lyon interact, and she could also provide some wise advice to them regarding their relationship.