One plot idea could be that Fem Percy discovers a new power that is somehow related to Hades' realm. She has to go to Hades for guidance on how to control it. This would lead to a series of interactions where they gradually gain respect for each other. For example, Hades could initially be skeptical of Fem Percy but as he sees her determination, he starts to help her more sincerely.
Another idea is that there is a prophecy that only Fem Percy and Hades together can fulfill. It could be about restoring balance between the mortal world and the underworld. Along the way, they face many challenges like battling ancient monsters that have escaped from the underworld. This would force them to rely on each other's strengths, Fem Percy's courage and combat skills, and Hades' knowledge of the underworld and his dark powers.
A third plot could be set in a situation where the gods are in chaos. Fem Percy and Hades have to form an alliance to stop the chaos from spilling over into the mortal world. They might have to negotiate with other gods, and in the process, their relationship evolves. Hades might start to see Fem Percy as more than just a demigod, as a key figure in maintaining order, and Fem Percy would learn more about Hades' complex nature and his reasons for his actions in the past.