I really like the author vs ghostwriter idea, but it'd be hard if you're limiting yourself to the books/Animorphs alone. Don't know how much info there is about how that was handled outside a few forum posts on the web.
The obvious thing would be how Animorphs deals with the morality of war: reasons for fighting; how ethics changes over time/circumstance; justifications of killing/slavery; the notion that all of one side is good or evil.
Beyond those ideas, I could see writing about portrayal of animal minds, specifically short comings or misinformation in the books presentation of such (biologist).
There's also the representation of fictional beings, how they were unique, or approriated from standard story telling models; the "noble savage" of the Hork-Bajir; the honorable but overly righteous Andalites; the Yeerks, particularly in regard to their tyrannical and war driven society; the sentient species full enslaved by it's own instincts, Taxxons (particulary unique imo); pacifist Pemalites; the joyful nature of Howlers; those tiresomely complex, adaptable and unbreakable humans with their spirit.
I think for any topic you choose those, it has to be decided if you'll present on the concept as executed by Animorphs (eg author vs ghostauthor: a literary examination with Animorphs as the examples) or how the Animorphs utilized the notion (the effects of author vs ghostauthors on the development and story of Animorphs). Similar, but different.
But I don't know, i don't do literature