Another plot idea is that they are involved in breeding magical familiars for young wizards. They aim to breed familiars that are more attuned to the specific needs and personalities of the young wizards. Harry and Lily could travel to different parts of the wizarding world to find suitable familiars to start the breeding process. They would need to study the behaviors and characteristics of different familiars, and use magic to ensure a healthy breeding process.
Perhaps it could involve them breeding plants with magical properties. They might be researching how to cross - breed certain magical plants to create ones that can heal more serious ailments or have other useful functions. For example, they could be trying to breed a plant that can not only heal physical wounds but also mental traumas in the wizarding community. They would have to search for the right magical ingredients, find the perfect environment for the plants to grow, and use their combined knowledge of magic and nature to make it a success.
One idea could be exploring a magical breeding project they undertake together in the wizarding world. Maybe they are trying to breed a new type of magical creature with unique abilities.