I like original stories and characters, so I think number two would be my choice. That is, as long as it is a completely new story, not just different characters who happened to walk into the construction site. I think something like a story about another group of morphers, in a different town, that is being invaded at the same time as the original animorphs' city, could be a really neat idea. Especially since you would be wondering the entire time why you never heard about them after the war was won. DUN Dun dun.
Good luck if you decide to write something.