I think the only chance of adding extra races will be if they do a serious follow-on DLC, because they'll want to get so many of the voice-actors back anyway that it might mean adding reaction dialogue for them would make sense. I still think the odds are kind of low, though.
As you say, adding Tav/Durge voices should be pretty trivial. It'd be really nice to have, too. Right now we have like, four very middle-class, very English voices for each gender. All of the female ones sound like people in their 30s-40s, specifically, and most of the male ones sound like 20s-30s. It's really quite a narrow range. Especially given Tav/Durge doesn't have that much to say - none of the voices have unique lines. I'd to to see similar ones to you - a gruff, maybe Cockney or Estuary one, a Scottish one (preferably an actual Scot not a bad impression and probably not a Glaswegian, god bless my relatives but...), a cut-glass fully-posh one (but well-spoken rather than the sort of mumbly-posh voice a lot of actual aristos - including royalty - and people called "Biffy" and the like speak with), maybe a nice smooth Welsh one to go with our dear Halsin and so on. Just more distinctive voices, not like "middle-class Received Pronunciation 30-something crew".
Faces/hair/beards I'd love to see more of too. Please some cooler beards - like give me Hawke's beard for god's sake - I can make a very good Varric already, but a Hawke? Neither wide-enough male faces nor sharp enough beards are available. Obviously we need in-game appearance customization too, but I would bet money that'll be here within six months at the latest.
Re: the Hells not-in-game content "spoiler": There's an entire partially-finished Avernus section, replete with the NPCs, monster, dialogue for the companions, and so on, in the code. So it would certainly make sense to build that out.