I think they could make another book. The Elemist can step in and make it one final battle. Rachel and Cassie and Ax are all allowed back in it. But in the end Rachel must go back. Bear, Wolf, Tiger, Gorilla, Hawk, Andalite. And they face off against 6 equivalent evil aliens. They fight one last battle. Who knows if anyone dies in it, but it would be one helluv an exciting fight. And then the Animorphs win in the end. Ax returns to Earth, retired from combat as a war hero on the Andalite home world. He morphs human to eat snacks, but still goes back to Andalite within the 2 hours. Jake is still the group leader. He gets over the depression, kicks Cassie's new BF's butt, and takes her back as his wife. They have a family. Marco has a family with some blond girl who likes him for his fame. They all get together once a week to remember Rachel and hang out at the mall. Happilly ever after stuff. That is what I want to see!
BTW, I am a lifetime Animorph fan. Read em all one by one as they came out. From 4th grade through the summer before 10th. Or something like that. And I absolutely Loved em. I cried and cried and cried when it was all over. I felt empty. I couldn't believe the ending. I wanted so badly for more books. I wished I could be a part of them.