A dynamic you wish was explored more in canon?
This could be any relationship: platonic, romantic, familial, enemies, etc.
I personally would have loved to see more about Victor and Mr Sweet's past, how Sweet got involved with the society, if Victor took him under his wing or not, etc.
I'd also have liked more development of specifically Rufus and Sarah. There's so much to unpack with them, but they really don't give us much beyond just telling us in passing that Rufus' parents adopted Sarah, them being both brother/sister and Osirian/Chosen One, and there being some sort of falling out.