When the now much despised Konami allows a wonderful Castlevania show to be done, it's hard for me to picture why Nintendo wouldn't allow the same to happen for one of their IP's which would only benefit them, though am not ruling anything out. But if Star Fox is anything to go by now, they're more open for animated things being done for their IP's, and maybe they'd have an involved helping hand in an animated Metroid being done.
For the Castlevania show, I believe it was a little mix of all three (origins, new content, mixture from the games), so I'd be open to whatever for Metroid as far as that. The first episode would most likely cover the beginnings of Samus's past with the pirates and Ridley (bound to be my favorite episode like the 1st episode of Castlevania with Dracula's story), while the next episodes would either show their own spin on what happened during some of the games (like how the beginning of Other M showed a cutscene of Super Metroid's Mother Brain) or some new content with Samus and take it from there.
Yeah, I'm not too worried about the voicing, since even Other M Samus's voice would've been fine if not for the writing and direction that had been taken with it. That approach there with Ridley would be the safest option, though I don't doubt either that a good english voice could be found for him. Would have to see what they do if the desire and plans for doing a Metroid show takes off.