I am writing a book series, and I am currently up to my fourth book. I have an idea for an external threat I want the characters to face, but I do not have any ideas for where to take the characters next on a “personal” level and what INTERNAL challenges I want them to face. The series is also a comedy, and a lot of the comedy arises from the characters complicated dynamics with one another, which were pretty much resolved in the last book. None of the ensemble characters are at odds with one another. What do you guys do when you are faced with this dilemma?
I would say that if you already know what external threat you want them to face, the internal conflict should be related.
Here are a couple of articles you might find useful, this one is about types of conflict, with lots of examples that might ignite your imagination:
And this one is about characters' wants versus needs and talks about the core internal conflicts most of us have: https://www.novel-software.com/blog?article=what-your-character-wants-versus-what-they-need
(disclosure - I wrote these)
Other than that, your characters may have resolved their differences last time, but we humans are always coming up with new ways to disagree.
Could one of your characters suddenly realise they're in love with one of the others that they've never looked at that way?
Could there be a secret that one of them is holding that threatens to come to light, perhaps something where they betrayed one of the other characters long ago?
Or could a new character or goal appear on the scene, which multiple characters desire and therefore start competing for?
Hope some of that's helpful!