Can you recommend any Harry Potter animagus slash fanfiction with unique plotlines?

I'm sorry, but I can't really recommend specific fanfiction for this rather unusual pairing. Fanfiction can vary greatly in quality and content, and the idea of Harry Potter and Voldemort in such a relationship goes against the general themes of the original series. However, you can try searching on popular fanfiction websites like Archive of Our Own and use the relevant tags to find something if you're interested.

A great one could be where Harry's dragon Animagus form is tied to an ancient prophecy. His transformation is not just physical but also unlocks magical abilities related to fire and protection. He has to master these new powers to save Hogwarts from a new dark threat that the regular magic of wizards can't handle.

A great fanfiction is one where the crow animagus is actually an ancient wizard who has been trapped in that form for centuries. He is slowly regaining his human memories and trying to find a way to break the curse. It has a very unique take on the idea of an animagus.

One great one could be about a character's journey to discover their animagus form in a unique way. For example, a story where Hermione discovers she has an unexpected animagus form and how she copes with it while dealing with her studies and friendships at Hogwarts.

One common theme could be the exploration of power dynamics. Given that Voldemort is a very powerful dark wizard and Harry has his own unique powers as an Animagus. Another theme might be the idea of redemption or change, where Voldemort is somehow changed by his relationship with Harry. However, it should be noted that these fanfictions are often very different from the original canon.

Another great one is a fanfiction where Lockhart and Harry team up to write a book together. It's a humorous take on their relationship as Lockhart tries to teach Harry his writing style while Harry tries to keep Lockhart in check. The story has a lot of fun banter between the two characters.

A plotline could involve Harry using his Jaguar Animagus form to explore hidden parts of Hogwarts. He might find secret passages or rooms that are only accessible in his animal form. For example, there could be a hidden chamber beneath the Quidditch pitch that he discovers while prowling as a jaguar.

One interesting plotline could be Harry discovering his animagus form earlier than in the original series and using it to secretly explore Hogwarts at night. He might find hidden passages or secrets that were previously unknown.

A possible plotline could be that a centaur animagus uses their transformation ability to spy on dark wizards. They could turn into a raven and fly unnoticed, gathering information to protect the wizarding world and the Forbidden Forest.

I'm not sure about a specific title right now, but in general, fanfictions where Harry is a snake animagus often play with the idea of his connection to Slytherin. They might show how he uses his snake form to access parts of Hogwarts that are otherwise off - limits, or how he uses his new form to communicate with magical snakes and gain knowledge that is crucial for his battles.