I would like for her to release maybe a new series, with with animorphs in it, but not with the same characters. I think that's how Warriors was done. I think the main problem is that with the Andalites in the end giving everyone technology and the secrete being out it would be hard to do. I have an idea though, but I'm scared that if I say it K.A. won't use it. Though if she does read this I give full consent for her to do so. Since the morphing tech is declassified only to a few people on earth it would be interesting to see what would happen if another group of kids were given the power because maybe the yeerks weren't all defeated. Who would believe that after the first defeat of the yeerks that they would come back and invade again in a similar manner? Furthermore, you don't have to worry about the animorphs from the previous books coming back because they are lost in space.
This reminds me about what they did with Ghostbusters. There was the old ghostbusters with the four guys, but then there was a new ghostbusters with some teens, and one guy from the old ghostbusters. I believe it was Egon. Since Cassie stayed behind, there could be a new installment with a new group of teens, plus Cassie. Cassie would be like 30+ years old, and serve as kind of an advisory role in the group. Like more of a secondary character than a main character.
Or just leave her out altogether. Personally I would have rather seen the old Animorphs go with the Free Hork-Bajir to save their homeworld than make a new installment with new Animorphs.
Something cool might be to have some Human criminals secretly break Visser Three out of prison. They want to use him to rule the world, but he winds up taking over the organization. Visser Three in a Human bdy, building up a criminal orginazation to attempt a new takeover of the world. Except his presense is almost unknown. His underlings will call him by his host's name, and such. He would be like Lex Luthor or the Kingpin from Spiderman.