The reason Maddie isn't as popular/embraced/liked by the fandom is partially because of the point you made about the fact there are no new characters and the story is going stale. Maddie is somewhat in 'intruder' on the dynamic, even though she is the daughter of some of the main characters. All the dynamics and character personalities are well established, and in comes the random girl who is kinda 'forced in'. If there were more new characters, this wouldn't have been the case, but now she's the odd one out (you and me, just us and your friend Steve). Give this girl some fucking friends/enemies/(for god's sake, maybe even a love interest) who aren't her semi-aunt/uncles or her parents. Ingrid doesn't count, she's a servant (could've been an option for a new (central) character, but alas, she's not been doing anything sadly).