Their relationship can be developed through shared adventures. For example, if they go on a quest together to retrieve a powerful artifact. During this journey, they have to rely on each other's skills. Cid's combat and strategic skills combined with Rose's knowledge of the local area or certain magic can bring them closer. They start to respect each other more, which is the first step in developing their relationship.
One plot idea could be that Rose discovers Cid's true power in a moment of crisis. For example, when facing a powerful enemy that the Shadow Garden can't easily defeat, Cid reveals his real capabilities to save Rose and the others. Another idea is a love triangle where Rose has feelings for Cid, but Cid is so focused on his shadowy plans that he doesn't notice at first, while another character also vies for Rose's attention.
You can try looking on fanfiction websites such as Archive of Our Own. It has a large collection of various fanfics, and there's a good chance you'll find alpha x cid fanfic there.
One good alpha x cid fanfic could be about their adventures together in a fantasy world. They might face various challenges and their relationship could grow stronger through it.
The Eminence in Shadow light novel is finished. It provided a conclusive ending for the characters and their adventures, leaving fans with a complete narrative.