The Dursleys could also be antagonists in a way. If Draco comes to their home with good intentions, they might not believe him and be hostile towards him. Their over - protectiveness of Dudley and dislike for anything magical could make them oppose Draco and any interaction he tries to have with them.
One possibility is that Draco, being from a pure - blood wizarding family, could try to 'educate' the Dursleys on the wizarding world. He might brag about his family's wealth and status, which could lead to some comical situations. The Dursleys, on the other hand, might be completely unimpressed and tell Draco off, which would be a new experience for him as he's used to being fawned over in the wizarding world.
Maybe an old family acquaintance of the Blacks who has a vendetta against Andromeda for leaving the family and is now worried that Draco, as a young Slytherin from a prominent family, is being led astray by her. This character could use their knowledge of the Black family secrets and connections to create obstacles for Draco and Andromeda as they try to build a relationship.
Well, in such a fanfiction, Draco might be completely out of his element when he meets the Dursleys. He could be shocked by their rudeness and lack of magic knowledge. Maybe he'd initially look down on them but then gradually realize there's more to them than meets the eye.
One interesting plot point is that Draco, being a Slytherin, might try to use the Dursleys for his own gain at first. But as he gets to know them better, he realizes that they are more complex than he thought. Also, the cultural clash between the wizarding world Draco comes from and the Muggle world of the Dursleys can lead to many humorous and thought - provoking situations. For example, Draco's confusion over Muggle appliances and the Dursleys' bewilderment at Draco's magic.
Obviously, Teddy is one of the main characters. He is likely to be the one driving the story forward with his actions and reactions.
In addition to Harry and the Dursleys, there could be some Hogwarts professors involved. For example, Professor Dumbledore might play a role. He could be the one who arranges for some sort of exchange or interaction between Hogwarts and the Dursleys. Also, Hermione and Ron could be main characters if they accompany Harry to meet the Dursleys or are involved in the events that bring Hogwarts to the Dursleys' doorstep.
Obviously, the Dursleys themselves are the main antagonists. Vernon Dursley, with his short temper and extreme dislike for anything out of the ordinary, could be the driving force behind Harry's death. He's always been shown to be cruel to Harry in the original series. Petunia Dursley, too, could play a major role. Her envy of her sister and her hatred for Harry could lead her to either directly cause his death or be complicit in it. And Dudley, with his spoiled and bullying nature, might also be a key figure in Harry's demise.
Most likely, the main characters from the next generation of the wizarding world, like the children of Harry Potter and his friends. And of course, the Dursleys.
Remus might interact with the Dursleys in a very calm and collected manner. He would probably be polite, despite their rudeness.
Well, Daphne in the 'Daphne meets the Dursleys fanfiction' would probably interact with the Dursleys in a very cautious way at the start. She would be observing their behavior towards Harry closely. Once she realizes the extent of their nastiness, she could try different approaches. She might try to reason with them, explaining how wrong it is to treat a child like that. When that doesn't work, she could use her magical abilities to disrupt their daily routine in a harmless but annoying way, like making the TV constantly change channels or the kettle never boil. This would be her way of getting back at them for their treatment of Harry and also a way to show them that there are forces they don't understand.