Let's see. What do I think about this?
1) Tobias should have been given a much larger role in the series. I mean, apart from his "eyes in the sky" bit.
2) In addition to the above: Tobias should have been given some sort of cool battle morph, aside from Ket Halpak.
3) Most importantly of all; a better return for David.
4) A confrontation between Esplin 9466 Primary and Esplin 9466 Lesser (a little sibling rivalry is always interesting, no?)
5) Have Tobias explain a bit more about what it was like to morph Taylor (not anything extreme, but you know...)
6) Leave out anything with the Helmacrons...usless little buggers.
7) Explain the Kelbrid, The Anati, and the One (who introduces more plot devices in the last book? An incompetent person!)
Have Tobias get his human body back at some point (Sort of ruins his whole character...but still.)
9) Should have been a Taxxon chronicles book.
10) Explain the origins of the Drode.
11) Explain the origins of Crayak
12) Explain the origins of the Time Matrix
13) Explain Sario Rips
14) Get minor characters like Jordan and Melissa more involved.
15) Dont have Rachel enter Marco's body in the Journey.
16) Explain the relation of the Garatrons (if any) to the Andalites.
17) Don't kill the Auxiliaries (at least, not James)
18) More about the Council of Thirteen
19) Give us the name of Tom's new Yeerk
20) Show us more Vissers