I'm confident that Blizzard will not kill Sylvannas off completely... she's a huge money maker for them and it just wouldn't be a good idea. They didn't build up Lillian Voss enough for us to be okay with her taking Slyvannas's spot atm and I doubt they could create a huge passion for Voss in such a short time. However, her description on legion says basically it's going to be a choice of her people or her... Well Blizzard wouldn't just say "Sorry guys not allowed to be forsaken any more" so it would be more likely that something does take Sylvannas out of the picture... however I still don't feel as though it would be permanent. I could see some sort of resurrection story or imprisonment story of some sort over a straight up death. Anything can really happen and we don't know what the Val'kyr are getting in trade for taking Sylvannas's spot in "hell". Perhaps if we can figure out what benefit they have in taking her spot, it could hint at possible expansions with her involvement. I don't think they'd do a Garrosh story with her becoming the big bad boss... that's been done and although Blizzard is not scared of recycling stories, if they produced anything more than a hint that Slyvannas will be turned into a garrosh-like character Blizzard would reverse it within hours of announcement as subscriptions / stocks and more would plummet.
It makes since that Tirion either dies or passes the baton to us in Legion (baton being Ashbringer). I mean Tirion was around back in the days of Uther and Arthas as paladins of the Silver Hand along with himself. He's fulfilled any of his purposes in resolving that story. I'm not sure if Tirion will die or if he will simply become too weak to fight and pass off the Ashbringer or something... It would make sense for him to die with honor but after defeating LK I don't see him just dying to a random infernal. However, we know we'll get ashbringer in legion and probably pretty closish to launch, so perhaps first big addition patch is when Tirion falls in battle in the form of a raid or something. Just my speculation.
Varian is going to die. Nobody likes him / has liked him for a long time. He had some plot relevance in the Lady Prestor lore but since then he's just been kind of going with the flow and Anduin is a much more beloved character in comparison.
Since players get Doomhammer, something has to be of reason why it's disbanded by its owner. I doubt death will be the cause of this one but more of a passing of the baton. With this being said, I think it would kind of rule out the similar option for Ashbringer and hints more towards Tirion dying in battle... Again anything is possible as far as I'm aware. I have yet to look into any blizzcon info from this year and I'm sure some speculation on these deaths were hinted at.
Again this is all speculation and as with everything, it's always best to sit back and watch the information be relayed as wasting time trying to crack the hints and guess can result in frustrations and incorrect information. I'd say speculate all you want, but really the only information that can be discussed is the Legion bios page, the gamescon dialogue and now thankfully we'll have this blizzcon dialogue which I'm sure clears up a lot of questions.
I mean I'm sure some people lost their minds when they saw the model for Xavius and how it hints at an emerald nightmare raid / dungeon or possible seed for future expansion. It doesn't make sense to have an emerald nightmare zone without a raid or dungeon, and it would make more sense to have a contentless emerald dream zone over an emerald dream raid or dungeon unless it has to do with a rescue of the emerald dream, so I'm going to say we'll see an emerald nightmare raid or dungeon with possibly some slight zone affect similar to the sense of a portal to the emerald nightmare but not much more and perhaps in the future players will finally get an emerald dream to roam around like they had wanted for years.... (even though it would be completely pointless and empty of all structures)
Just a few points of my perception on things. I'm very excited to get the information from blizzcon. I really hope they reverse the decision that the twisting nether spands all universes / dimensions / realities as that completely destroyed a lot of the lore and produces more issues than it resolves.... however that information was simply from a tweet.... if that tweet did not exist, Lore would not be so messed up right now.