This question is very similar to this one, but I felt it was distinct enough that it was worth asking as a separate question.
In a lot of fantasy and science-fiction stories that are nominally set in the present, day, in the first volume or so there is almost invariably a plotline where the everyman main character is drawn into the supernatural world and is forced to adapt in some fashion. Quite often this takes the form of a typical Campbellian hero's journey. Getting dragged into the supernatural world is crossing the threshhold, learning about the world is challenges and temptation, they hit a low point as an abyss and then adapting to the supernatural world and/or returning to reality is the return. Through the process of this adaptation, we get ready-made conflict as well as a way to learn about the supernatural elements of the setting as the audience does.
However, what does one do to continue to develop a character and keep them interesting after this initial arc plays out and they are familiar with the supernatural elements of the setting? I realize a lot of this is dependent on the character and hence is highly subjective, but I am trying to figure out what to do to keep these characters interesting in a general sense. Once the character has played out the initial hero's journey cycle, their initial source of conflict is gone, and it's not clear how to develop a character from there. I.e., descending into the supernatural underbelly of the world isn't as thrilling the second time around as it is no longer the great unknown to the character and the character may have friends or allies from the first time around to help them out.
I've noticed in other urban fantasy works (e.g., The Darren Shan Saga, Harry Potter, Gregor the Underlander, etc.) almost invariably pivot into a "someone wants to take over the world" plot when they initially run out the warranty on the initial "character discovers the supernatural world" arc, or else they draw out the "adapting" arc over multiple books. However, a lot of times these kinds of arc feel hollow because unless the bad guy is written really, really well they don't actually further the character development of the protagonist, merely represent an end-boss threshold guardian to overcome. E.g., one of the big reasons why Umbridge was considered a more memorable villain than Voldemort, Umbridge was more of a personal challenge to Harry whereas Voldemort was just a big, bad snake man who shot green lasers that Harry had to defeat to end the conflict. Voldemort clearly hates Harry but Harry just seems resigned to fight Voldemort out of duty (as seen in the conversations at the end of Order of the Phoenix) and the antagonistic relationship produces little development in Harry (especially compared to, say, Sirius' death). This kind of plotting swerve by itself seems like a bit of a problem because the pivot into the "bad guy wants to take over the world" plot doesn't seem to provoke more character growth in the protagonist, and the protagonist ends up stagnating outside of subplots.