Perhaps Tonks gets involved in a mystery related to a Black family heirloom. It has some sort of magical power that the wrong people are after. Tonks has to use her shape - shifting abilities and her wits to protect it and keep it out of the hands of dark wizards who want to use it for evil purposes.
A plot could revolve around Tonks forming an unexpected alliance with a member of the Black family, like Sirius Black's younger cousin who has different views from the rest of the family. Together, they try to reform the family's reputation within the wizarding community.
In some fanfictions, the Black family might initially be wary of Tonks due to her 'blood traitor' ways. For example, Bellatrix could be extremely hostile towards Tonks, seeing her as a threat to the pure - blood ideals the Blacks once held. However, Sirius might be more accepting as he has also broken away from the traditional Black family views. He could see Tonks as a kindred spirit in a way.
One interesting plotline could be Tonks trying to fit into the complex and often dark Black family traditions while still maintaining her own identity. She might struggle with the family's views on blood purity and her own more inclusive beliefs.
One idea could be exploring the Black family's history before the events in the original story. Maybe delving into their family secrets or hidden talents.
One plot idea could be that Tonks uses her Metamorphmagus abilities to help Harry in his undercover missions during the war. For example, she could change her appearance to infiltrate Death Eater strongholds with Harry. Another idea is that Harry discovers a new side of Tonks' personality that he didn't expect, and they bond over their shared experiences of loss. Maybe they both lost someone important to them in the fight against Voldemort and find comfort in each other.
One idea could be exploring the backstories of the Black family members. For example, delving deeper into Sirius Black's time at Hogwarts before his expulsion. Another could be creating a story where a member of the Black family tries to break free from the traditional pure - blood ideology and embrace a more inclusive wizarding world. Or perhaps a fanfiction about the Black family during a time of great wizarding war, showing how they coped and the different paths they took.
A possible plot is that Harry and Tonks time travel to a point where the creation of Horcruxes was just beginning. They could try to stop Voldemort from splitting his soul, which would change the whole course of the wizarding world's history. They would face many challenges, like not being discovered by Voldemort or his followers. Also, Tonks might use her skills to distract or mislead some of the Death Eaters while Harry looks for a way to destroy the Horcrux - making materials or the places where Voldemort hid them.
One plot idea could be Tonks helping Harry to master some of the more difficult Patronus spells. Since Tonks is a skilled witch, she could offer unique insights and training methods. Another idea is that they go on a secret mission together to find out more about Voldemort's hidden lairs. They might have to use Tonks' Metamorphmagus abilities to their advantage.
One idea could be the cultural and family clashes that come with such a contract. Maybe Harry's friends are shocked and have mixed reactions. Another could be how Harry and Tonks grow to understand and care for each other in the context of this contract. For example, they start off as strangers in this arranged - like situation but gradually find common interests.
One idea could be that Tonks discovers a hidden vampire community in the wizarding world while on a mission related to Harry Potter. Maybe the vampires are being wrongly accused of attacking wizards, and Tonks, with Harry's help, tries to uncover the truth.
One idea could be exploring how they prepare for the baby in the wizarding world. Maybe they have to find special magical items for the nursery. Another could be how Tonks' shape - shifting ability might be affected during pregnancy. For example, if she unconsciously shifts while sleeping due to hormonal changes.