Definitely Harry Potter. He was a good friend of Lupin and Tonks, so he could be like an uncle figure to the baby. He could visit often and share his own experiences growing up in the wizarding world, giving the baby some valuable advice.
Hermione Granger would also be great. She's so smart and resourceful. She could help Tonks and Lupin with parenting tips, especially when it comes to the baby's magical education. For example, she could recommend the best books for the baby to learn about different spells and potions as it grows up.
Sirius Black would be an interesting addition if you're doing a story that involves flashbacks or some sort of magical connection. He was Lupin's friend too, and his rebellious and loving nature could add a lot of depth to the story. Maybe the baby has a special connection to Sirius through some old family magic or something.