Animorphs started when #1 The Invasion was released in 1996 and ended in 2001 with the release of Animorphs #54 The Beginning. The series was about six teenagers fighting to stop an alien invasion of Earth. It also covered a lot of backstory from how the Hork-Bajir were all enslaved to how the Ellimist came to be. I admit not being a fan of what eventually became of the Ellimist, but that's a story for another day.
As the series was still going on, Nickelodeon made a live tv series that starred Shawn Ashmore as Jake, the leader of the Animorphs.
This article will be on the basis that K.A.Applegate, creator of the books, and Michael Grant are on the team of making another Animorphs adaptation. That Animorphs is already having movies and a tv show to cover the major plots of the books.
So I will talk about what things not explored in the books would be interesting to see on the big or small screen.
The two main types of Controllers, what hosts for the mind-controlling space slugs Yeerks are called, are Hork-Bajir and Taxxons. There was The Hork-Bajir Chronicles that gave the backstory of the species and why there are no free Hork-Bajir.
But there was no backstory like that for the Taxxons. There wasn't even a detailed backstory that you found out through a series of books. For the Hork-Bajir in #13 you learn that Prince Elfangor was telling the truth when he said the species was peaceful. But it's only in The Hork-Bajir Chronicles, one of the books outside of the main series, that you learn even more about them.
All you really learn about the Taxxons is that they're really hungry at all times and will both cannibalize and self-cannibalize. That they made a deal with the Yeerks for food but that some Taxxons resist. You learn about the Taxxon resistance first in The Andalite Chronicles.
But we never get what made the Taxxons that way or really anything else.
In the Animorphs Expanded Universe we could get to see about the deal with the Yeerks being made as well as the Taxxon resistance being formed. It'd be more than awesome to see such a split in the species.
Maybe even getting to see how the Taxxon home world was before the start of the Animorphs series. Thereby seeing why a species would evolve to have such an appetite.
Origins of Crayak
The Animorphs series does a great job at painting every side as a shade of grey. In the beginning, the Yeerks are evil and the Andalites are good. But as the series goes on you found out that not all Yeerks are the same and some are even against the Invasion of Earth. You also find out that Andalites are archaic with their beliefs. Even the Animorphs themselves are painted in shades of grey.
So it is a little odd that Crayak is shown to be the symbol of pure evil. Esplin 9466 (Visser Three in #1 but later gaining the rank of Visser One) is the main villain of the series but even he is given reasons other than 'I am evil' for what he does.
With this Animorphs Expanded Universe we can finally see Crayak in a shade of grey. While I'm by no means saying that he should become a heroic character, I am saying that finding out his reasons for doing what he does would make him a little more well rounded.
I believe this would best be accomplished by showing whatever Crayak was driven out of his galaxy by. Maybe this thing was more evil than Crayak? Or maybe Crayak becoming like he is seen in the Animorphs series was a gradual descent into madness?
The One's Origins
The final Animorphs book came and there were complaints. Some I don't agree with such as any complaints that deal with how depressing post-war is for our heroes. Animorphs painted a very real picture of what war is like.
War isn't something to be enjoyed, not everyone survives, and friendships made during the war can become non-existent afterwards. Cassie and Jake, a couple that seemed made for each other through the majority of the series, broke up by the end. The reason being is that war changes you and Jake did some...well...you can read that yourself as wow. Just wow.
But one major thing I agree with is the character of The One appearing and never being explained as the book ends before you can understand it. Applegate has mentioned that The One has never been mentioned in the series prior to the final book. This dashes fan theories about The One being the mysterious being in #41 The Familiar.
So in an Animorphs Expanded Universe we can finally learn about the creature known only as The One. Since so little is in canon about the being, there is much that can be done.
We can either have a stand-alone movie featuring The One's backstory. Maybe even connecting it to the power Crayak originally fled from. Or having it connect to #41 in that it likes to study creatures and run tests. Or about a million other things.
Or its history could be revealed over a number of movies and/or episodes. With the final reveal being held out until later. This way would allow fan theories to sprout up as everyone struggled to figure what was going on. And having a fandom interact like that gets people more interested in whatever you're doing.
The latter has been done in the Animorphs books with other characters and creatures:
Esplin 9466 is first met in #1 The Invasion and his history is later expanded in both The Andalite Chronicles and Hork-Bajir Chronicles.
Crayak is first introduced in #6 The Capture but who and what he is is only later revealed in #26 The Attack.
I know that the likelihood of another Animorphs adaptation is unlikely, but I can dream. And I hope my dream was interesting to you.