For once not an AU and I may write this myself someday. I'm still pondering the details.
Anyway here's the idea.
It takes place a year after the battle with Acnologia. Gray and Juvia are still getting around to their relationship status but now the rain woman has other concerns on her mind. For as long as she can remember Juvia has always wondered about where she came from, who her family was, and how she was born with the power of rain but the answers to all three of those questions have always been a mystery. That is until the group takes up a job in a land located in the mountains where it is always storming and the people are enslaved by a tyrannical thunder bird called Thoren who plans to use the forces of thunder, lighting, and rain to take over Fiore.
During their stay they learn that these mountains were once home to maiden-like water beings called Undines but years ago when they refused to unite their powers of rain and water with the thunder birds to carry out their sinister plan, their former leader (Thoren's father) had them all killed. However it is rumored that one managed to get away, an infant who was born from the union of an Undine and a human.
It isn't long before certain connections are made between Juvia and the Undines but when Thoren learns of her existence he abducts her as a means to carry out his wicked desire. Now Gray and Gajeel have to rescue her while the rest of the gang have to locate a stone that is needed to make Thoren's goal possible.
Of course there is going to be Gray and Juvia romance and Gajeel and Juvia brother-sister love in this fic if I write it.