I think about this a lot when I go running, actually.
If I had unlimited resources and the rights, I'd make a 6-part series. Like Star Wars.
I'd cast unknowns for the Animorphs and put in big-name actors for the smaller roles. Ralph Fiennes could be Visser 3, Penelope Cruz could be Visser 1/Eva, James Franco could be Erek, who the hell cares as long as they're big names that can draw people in but also good actors that can get **** done.
I'd exaggerate and flesh out the Animorphs' personalities to strengthen the conflict and update the story. Rachel would be the jaded cheerleader, Marco would be the internet geek, and Tobias would be the Ren McCormick/Holden Caulfield/Nick Andopolis.
Movie 1: Obviously includes most of Book 1, but compresses material from the other books. When they try to rescue Tom from the Yeerk pool, they get taken up to the Blade Ship to meet Visser 1 and they learn that the war is bigger than they thought it was (politics, etc.). Marco decides that he has a reason to fight and Tobias gets trapped.
Movie 2: They meet Ax and try to destroy the Kandrona. While they do this, they meet the Ellimist. Rachel first realizes that she's out of control. The Chee are introduced here.
Movie 3: Tobias gets his morphing power back when they form the Hork Bajir colony in the first half hour (hints are dropped in earlier movies). Then they discover David, who is the center of the movie. The main conflict is between Rachel and David who are more similar than they want to admit, but want different things from the war (David wants revenge, Rachel wants to save her family/friends/Tobias). There is a sub-plot about Rachel trying to force Tobias out of the war. (Erek has a smaller role in this movie.)
Movie 4: This focuses on the Iskoort homeworld, the Crayak vs Ellimist. A sub-plot involves Crayak tempting Rachel with David, but she resists. (Erek has a large role in this movie, obviously.)
Movie 5: Build-up to Movie 6. Marco frees his mom, Jake's parents get captured. After the battle on the destroyer (maybe that can be where they rescue his mom?), they all move into the Hork Bajir colony.
Movie 6: Visser 3 and Ax fight to the death, Ax kills him. Jake fights Tom but doesn't kill him. They still flush the pool. Erek is tricked into helping them and is angered. Rachel doesn't die and Tobias traps himself as a human to stay with he.
The timeline is a little wonky, but I think that includes the big things people want to see. It does justice to the Crayak/Ellimist story, I think most fans would want to see that in there. I also think David is important to have in there too, especially because of his conflict with Rachel.
Unfortunately I don't think the Taylor story arc could make it in, even though the scene where Jake and Visser 3 "face off" was one of my favorites (I think #33 was one of, if not the best, ghostwritten book.) It'll be hard to give Tobias' actor enough screen time if he gets his morphing power back kinda late, but he can't get it back too early. The scene where he turns back to human for a bit in #7 might work, but the producers could actually combine the Kandrona/Hork Bajir plots to create enough time for the David/Rachel plot in movie 3.
The big thing that I'm undecided about is how the Auxiliary Animorphs would be treated. They weren't handled well AT ALL - the characterization in #50 was completely ignored afterwards and they were just turned into a plot device - so there's no point putting them in there if the producers can't/won't do it right. However, I LOVE the idea of James being a foil to David (admit it, in a perfect, non-ghost-written world he WAS), so hopefully he's worked in there somewhere.
Animorphs is kind of an ensemble cast with 6, so except for Erek and villains, side-characters probably won't get much screentime. (ex: Rachel's mom or Cassie's parents could maybe give comic relief)
Do any of these ideas sound okay? It's a rough outline, obviously.