Well, obviously the 333s over time gave rise to the Five, which the Andalites think they wiped out but it turns out some bit of them survived and became the One.
I kid, but...maybe I don't
Pemalites, for sure, I never got a good visual on what they looked like, and of course it'd be interesting to see there history.
The Nesk, curious what they've been up to for the last 65 million years.
Whatever the species the Drode is, and Crayak for that matter. And the thing that kicked out Crayak way back when.
I always found it weird that we never got more of the Taxxon POV, that could have been interesting to see them separate from the Yeerks.
But above all, left to one, the Skrit Na. My god, the Skrit Na, where, when, why, what. Arguably the catalyst of every major event in the series, but I we ever get is "worthless species, the Skrit Na."