Maybe it's a group of fans who collaborate on the fanfiction. They might each bring their own ideas about what kind of familiar Harry should get and how it would change the story. They could share the writing duties and come up with a really interesting piece of fan work.
It could be an amateur writer who is a huge Harry Potter fan and has a creative idea about Harry having a familiar. There are so many fans out there who like to write their own stories based on the wizarding world.
Well, perhaps Harry discovers a magical creature in the Forbidden Forest that becomes his familiar. It could be a small dragon-like creature that only he can communicate with, and it helps him find hidden secrets in Hogwarts.
The main antagonist could be the Ministry of Magic if they view the succubus as a dangerous and uncontrollable creature. They might try to separate Harry from his familiar, and in the process, become the main force that Harry has to oppose.
Well, perhaps the succubus could initially be a mystery to Harry. She might seem like an ordinary creature at first, but then he discovers her true nature. Maybe she helps him in some unexpected ways in his battles against Voldemort, like using her unique powers to distract the Death Eaters.
Well, Harry could marry one of the veelas. Given their beauty and allure, it would add an interesting dynamic to the story. Their relationship could involve Harry having to deal with the jealousy of other wizards and also the cultural differences of the veelas. Or he could marry a descendant of one of the founders of Hogwarts. This would tie in a lot of Hogwarts history into their relationship, and their wedding could be a grand event that unites different factions within the wizarding community who have ties to the founders.
It could be Voldemort. He might use hypnosis as a new way to get to Harry, trying to make him a weapon against the Order of the Phoenix. Another possibility is a new dark wizard who wants to make a name for himself by controlling Harry Potter.
It could be Dumbledore. Dumbledore is always full of surprises and he might give Harry the staff as a last - resort weapon to fight against Voldemort. He could have been secretly researching the staff and its powers and decided that Harry was the only one who could wield it properly.
It could be Hagrid. Hagrid is always looking out for Harry and giving him things. He might come across this special trunk during one of his travels in the Forbidden Forest or in some wizarding market and think it would be perfect for Harry.
Perhaps a group of anti - dragon fanatics within the wizarding world could be the main antagonists. They see the dragon as a dangerous and uncontrollable creature and will do anything to get rid of it, even if it means going against Harry.
The antagonist could be a jealous Slytherin student. They might see Harry's new snake as a threat to their own status in Slytherin house. So they try to steal the snake or harm it in some way.
In theory, it could be an interesting concept in fanfiction. However, it would be extremely out of the ordinary considering the nature of Xenomorphs. They are highly dangerous and aggressive creatures from the Alien franchise, which is very different from the magical world of Harry Potter where familiars are usually more docile and magical in nature. But in the creative realm of fanfiction, the author could come up with some sort of magical binding or control to make it work.