That would most likely be a Tales of the Rebellion, though realistically; given the show's track record thus far, it seems mostly concerned with characters Filloni had a hand in shaping, so while not impossible it does seem rather unlikely that we'll get either Acolyte or Andor character specific entries. Don't get me wrong, if Headland and/or Gilroy want to come back and write/direct some animation then I'm all for it, but I wouldn't hold my breath, and I certainly wouldn't want them to do this kind of thing without them.
For Rebellion we're much more likely to get arcs for the likes of Sabine, Omega, Hera, etc. Sith would be a but more tricky since the pool is decidedly smaller. Vader seems like an obvious choice, but I don't know if they really want to do a whole arc using the AI voice, and recasting while totally doable (they've already done it in the video games) seems like a somewhat bigger step or animation for some reason. Plus, short of adapting material from the Charles Soule comic run (not the worst idea in the world!) there's really not much to say about Vader that hasn't already been covered in-depth.
Palpatine . . . I mean there are possibilities here, and there's apparently plenty of material in the vault thanks to the 'Star Wars: Underworld' show that never was, but do we really want this character demystified?
Maul is much more likely, even with his own show on the way as there's still plenty of gaps to fill (especially of his early life), plus whatever circumstance landed him on Malachor, and even the events of the unfinished Clone Wars arc (though I'd rather see that given a full adaptation as a feature length 'Son of Dathomir' streaming movie!)
Beyond that; Tyranus has already been covered, Plagueis isn't established enough to be able to tell any kind of substantive story in so short a time, and again that's straying into 'Acolyte' territory.
What I wouldn't mind is something more in the spirit of the show's namesake; an actual 'Tales of the Old Republic' anthology show where they're more free to do what they want; cherry-pick Legends characters, create whole new ones, expand on the stories of legendary figures like Darth Bane and Tarre Vizla, tell fable-like stories from the period that can vary wildly in style and content. Bit of a pipe dream, I know, and it'll have to wait until at least Mangold's project is locked (assuming it even makes it out of development.) But even more so than the High Republic, it's a very fertile period to tell all kinds of new stories.
