Perhaps Teddy would try to expose Umbridge's true nature. He might gather evidence of her wrongdoings, just as his father and his friends once did against the Dark forces. Teddy could use his charm and the respect he has among the students (due to his family's reputation) to get others to see Umbridge for what she really is - a power - hungry and cruel authority figure.
Well, one possibility is that Teddy, being the son of Remus Lupin, would likely be filled with a sense of justice. He might immediately be on guard around Umbridge. Since Umbridge was known for her cruel and unjust ways at Hogwarts. Teddy could use his Metamorphmagus abilities to play tricks on her, like changing his appearance to something that scares her when she's being particularly mean.
Well, Teddy might be shocked by the Dursleys' coldness at first. He could show them some magic in a very subtle way, just to see their reactions.
Well, Teddy could learn a great deal about his family's history from the members of the Order. Maybe he'd discover some hidden secrets or untold stories.
He might react with defiance. Teddy is not one to back down easily. If Umbridge tried to impose her unjust rules on him or his friends, he would likely oppose her openly. He could use his ability to change his appearance to make a statement, like turning into a wolf (in honor of his father) to show his strength and resistance.
Well, it could be about Teddy Lupin traveling back in time and meeting his father's friends, the Marauders. Maybe he gets to experience their pranks first - hand and learns some new magical tricks from them.
Teddy might try to have a friendly conversation with the Dursleys, but they would probably be hostile.
Well, aside from Teddy and the Marauders, there could be other characters as well. Maybe some of Teddy's friends or classmates from his own time who are somehow involved in his journey to meet the Marauders. For example, if Teddy discovers a time - traveling spell in the library with the help of a studious friend. But the core characters would still be Teddy Lupin and the Marauders. Teddy is the link between the present and the past in this story, and the Marauders are the ones with all the stories and experiences from their time at Hogwarts.
In this kind of fanfiction, perhaps Teddy Lupin discovers a magical way to time - travel and meets his father. It could show Teddy learning about his father's character and values through their interactions. Maybe his father teaches him some unique magic that was passed down in the family.
It could be a rogue Death Eater who escaped justice and is now seeking revenge on the Order and anyone related to them. So, Teddy, being Lupin's son, would be a target.
Well, perhaps Teddy would be shocked by the Dursleys' coldness. Teddy, being a more open - minded wizard, might not understand their narrow - mindedness at first.
It could be a dark wizard who is trying to prevent Teddy from meeting his father. This wizard might be afraid that Teddy will gain some powerful knowledge or magic from his father that could be used against him.