I've just been thinking about this (obsessed much? Yup, this may be the holy grail of Trek) IF the audio tracks still exist and can be found, then that's that. If it can be found and preserved, then even if it's 10/20 years down the line there is always a possiblity something can be done with this, be it an audiobook or CG movie.
I think a movie might be more viable than an episode (or a game I guess, funnily enough I never thought of that?!), given the material. It would give SOVF it's own platform. But imagine it? A brand new full-length adventure with the original cast. And Mark Lenard as Sarek. Obviously any new production would have to go through CBS Studios now rather than Paramount, and the actors/estates, writers and make sure the rights to the story are secured.
James Doohan, Mark Lenard and DeForest Kelley are no longer with us, so that would make the project extra special. Hopefully Kelley's lines would be good enough to use.
I know I'm reading into things a bit here, but I am an optimist. We just need someone in LA (presumably) who can track stuff down and go to CBS with it.
Again I don't see the point in dubbing in old audio clips of the TOS cast, for an animated movie or series, rather just creating a new audio track with a new script.
Trying to dub old audio clips of these actors would sound forced and stilted. No one besides hard core trekies would care that its DeForest Kelly's voice in the cartoon, the stilted way the voice acting was done trying to dub in long deceased voice actors would be a turn off to the general audience. If they are going to make an animated movie about Star Trek, have a new audio track, not an old one. I think a pre-lay track would sound way better then dubbing in old audio clips.