IMHO, an Animorphs movie should have, first of all, a huge budget. The special effects are essential, beacuse there are morphing, animals, aliens. fighting, spacecrafts, adventures underwater, in the space, on other planets, and, furthermore, the visual spectacle, if done well, could attract a lot of audience who doesn't know Animorphs.
The main problem of the TV shows was the budget. Poor special effects. Aliens looked like puppets, spacecrafts were improbable, the enviroments were limited (e.g. they could not realize the recovery of Ax underwater but in a container or something similar).
Then, the plot. It's impossible to reduce the whole story in one film, but on the other and it's impossible to represent all the books. It could be a trilogy or a tetralogy. They may not consider, obviously, Helmacrons, Nartec, book 32, 39, 41 and others.
About the cast : I don't know who could play the role of the Animorphs, I don't know teen actors now. The idea of Emma Watson for Rachel is good. I have no idea about the director - maybe Joss Whedon or Steven Spielberg.
Another crucial thing : the screenplay should be very suitable to make the story flowing, as the single books are self-contained. (except for David Trilogy and the last two).
