I LOVE this idea!! And I always agreed with the thought, that Catra COULDN'T have been the only villian in the first series.
And while I 100% agree with your Wave 2 and 3 choices, I would have to change around your 1st wave just a bit.
Catra - Obvious main villian
Double-Trouble - Totally a villian... Trouble is in her name for Pete's sake. How many heroes have Trouble in their name... C'mon.
Castaspella - Even when I was little, her name just sounded very chilling, and villianous. She'll cast a spell on ya; She's a witch of some sort!! Villian for sure!
Now, where I differ... I'd trade Glimmer for Frosta. Going back to names alone, Glimmer is a very fun, and whimsical sounding name. Not exactly evil sounding... at all really. Even her powers scream heroic warrior to me. Not to mention the purple and silver color scheme of her costume. Now Frosta to me seems more like a villian. Cold, bitter, closed off from warm beautiful things in life; surrounded by cold, and snow... nothing grows, or come to life in the snow and ice. And the dark blue and icey white color of her costume; She is the Evil Ice Empress of Etheria!!! Catra's right hand woman in my opinion.
There ya have it... my 2 cents.
Great topic by the way!!