Wow....i love this topic because there are a bunch of ways you can take it.
Basically four.
1. continuation of the series
2. Another megamorphs
3. another Chronicles.
4. another series that occurs at the same time as the original Animorphs. Like another group of kids (maybe they wouldn't have morphing power, maybe something else) somewhere else, like the east Coast, Europe, or...Australia! haha.
There are simply so many ways you can take the series because there are so many aspects we don't know yet about each alien race.
Like someone said we can have a series that deals with the relationships of Humans with Andalites. It could show them picking off the remaining Yeerk resistences as well as the trials of the members of the Council of Thirteen. Yes, realistically, it won't all be happy and smooth. In fact, I can see the remaining Yeerks play off the power vacuum by pitching the Andalites against the humans a la "Sum of All Fears" style. The Yeerks would just be working for the One...or the Kelbrids. Cassie staying on EArth would be all right because she could be the Human viewpoint of things back on Earth. The dangerous thing with this plot is that it could quickly turn into another "Ender's Game" or "Stargate" like series. I guess what made the Animorphs series so awesome was it's different setting, the secrecy, the kids, the smaller scale invasion.
The other thing is this. We don't know that much about the One at all. Ax probably really isn't dead, and while there isn't a loophole, there are a lot of things we don't know about the One. Perhaps his Assimilation of races is not how we understand it to be. Of course....we don't really know anything about the Kelbrids either. We could kill all the Animorphs except Cassie and Tobias and start with something new?
I would like to see a Pemalite Chronicles dealing with Crayak.
or...the Gedd Chronicles! hAHAHAHA.