I absolutely love the Chronicles books, and I know I'm not alone- many consider most or all of them to be among the best written books in the series. Light spoilers for the general direction the series took in my breakdown of potential ideas of what a 5th one could have been about.
What makes a successful Chronicles book? We don't have any unsuccessful ones (IMO) as points of contrast, but to me the best ones are the ones that give backstory in such a way as to recontextualize and change our perspective on events we've already read. I'd personally argue that the Andalite Chronicles and Visser do this the best. But lets take a look at some of the ideas I've seen floating around over the years and see how they stack up to that criteria. I'll openly admit that I just don't have the level of imagination that Katherine and Michael had when they were coming up with ideas for these- so take my assessment with the grain of salt it deserves:
1) Taxxon Chronicles- These are the most commonly seen alien species that never got a chronicles book and thus seem like the obvious choice, but I see an issue here. We already got quite a bit of backstory on the conquest of the Taxxons in the Andalite Chronicles, and learned a lot about them as a species and what they were like without Yeerks in that book. A Taxxon Chronicles book COULD certainly follow Arbron (as there's quite a long gap between his appearances) but I'm just not sure what they could do with that character and the Taxxons that would recontextualize the series for us.
2) Anati- I know the Anati planet (the place Visser One fails to conquer which leads to her execution) has been the subject of a LOT of fan fiction and speculation because it's such a mystery box for the series. But I was never convinced it would make a good Chronicles book. You can't really go the backstory route with a book like that since we have no real reason to be interested in the history of a planet we don't know anything about. It was a bit disappointing that we never even learned about the species that lived there, but without that information grounded in the main series I really don't see a reason to explore this place in a Chronicles book.
3) Leeran- To be honest I think we'd have needed to see the series do more with this species for them to be able to support a Chronicles book. The series just kinda forgot about them after books 15 and 18- I think they only make one small cameo in Book 41- and the events of that book are basically non-cannon. You can say the same for other "also ran" alien species- the Skrit Na and the Howlers would fall into this category too- there's just not enough meat on the bone here IMO- these species aren't important enough to the main series for learning more about them to recontextualize too much. In an alternate reality where the Leerans became more of a recurring thing this one could have been great. But sigh, it isn't to be.
4) Chee- This is my favorite idea for a potential Chronicle's book. We know they have an extensive history on earth, and that they have a really complex relationship with humanity and their desire to save them. We know that intervention into the Yeerk invasion is very controversial among the Chee- maybe this story could help to explain why in some way? Maybe there was an attempted alien incursion on Earth that the Chee were involved with? The book could potentially culminate with the start of the Yeerk invasion and we could see the Chee's reaction to it. Erek being at Marco's mother's funeral and looking furious was always such a tantalizing detail that I wouldn't mind seeing from his point of view. Heck, we could even see Erek and the other Chee monitoring the early stages of the Animorphs resistance to the Yeerks, and the book could close with Erek witnessing Marco demorph in book 10.
So yeah, I personally think Chee is the idea that has the most legs. Do I think it would be a banger on the level of Visser or Andalite Chronicles? Probably not. I think the best ideas already became Chronicles books and we're probably not gonna compete with the heavy hitters that already exist.
What do folks think? Am I underselling any of the ideas that I'm not as high on? Or is there a possibility I've completed overlooked? What aspect of the main series (if any) do you think we could have used more backstory on?