Roddenberry went with Filmation instead of another animation studio is because they were the only ones who didn't want to turn it into a formulaic kids' show with teen sidekicks and cute alien or robot mascots.
Yes, we could have ended up with STAR TREK CADETS!!
however it was Filmation's Norm Prescott that broached the "Star Trek Cadets" TV show..When Roddenberry refused to give up control of his baby to Paramount it was morphed into what the show actually became.
the concept later was used in what became "Space Academy".