I think recasting the classic Star Trek cast for animation would be a huge mistake. It's unnecessary too. There was an unproduced video game written by DC Fontana which they all recorded dialogue for many years ago.
There is one guy out there that unlocked a ton of hidden recorded audio out of the games and came up with a really ingenious system of creating new audio clips and phrases using word fragments in the actor's own voices. He claims he could create any dialouge needed, if someone was animating, if I recall correctly.