Actually, if Sylvanas had really wanted, she probably could have survived…and this is valid for King Anasterian, too. Just stay away from Arthas and the bulk of the Scourge when it was clear they couldn’t be stopped, jump on a ship, hide in the forests of the Ghostlands, teleport away…sadly the rules of war of the High Elves about honor -that back then were pretty similar to the rules of the Humans and the Alliance- forbade the major leaders to run away in case of an enemy invasion, they had to stay and protect their people even at the cost of their own lives, King Anasterian himself fought on the battlefield wielding Felo’melorn during the Troll Wars alongside his troops for example. So they died against Arthas and the Undead…
but if Sylvanas survived, I think she wouldn’t be the Ranger-General anymore obviously because of her failure in the invasion, but I don’t really think she would be appointed as “Regent Lady of Quel’thalas”, either…that would still be Lor’themar, probably.
I think Blood Elf Sylvanas would want to take revenge against the Scourge at all costs, and help the Sin’dorei with their addiction problem, so I’m almost sure she would join the expedition of Kael’thas in Lordaeron (and then Outland with Lady Vashj and Illidan) as Kael’s second-in-command.
So she would become an Illidari and possibly, if she survived the failed attack of Illidan, Kael and Vashj against the Lich King in Icecrown, maybe she would have become a Sunfury later, too…which as we know, it could have been very bad for her
but if she survived even that and returned to Quel’thalas, redeemed and forgiven like a portion of the Sunfury have been, she could actually become the Regent Lady, if Lor’themar accepted to give her his position eventually, considering he didn’t really want the job as well… In any case it’s clear she would be one of the major leaders of the Sin’dorei like Rommath and Halduron, obviously.
Although it’s difficult to make predictions about what exactly would have happened because if she had survived the story would be quite different not just for the Blood Elves…I don’t think the Forsaken would really form as a faction without her personally freeing them from the will of the Lich King and leading them as a Dark Ranger…
Also Garithos’ army would survive so maybe the Alliance would still have a small chance to reclaim Lordaeron, because she was the one giving the order to kill them all after they both took Capital City from the Dreadlords. And without the Dark Lady ruling the Forsaken, the Blood Elves probably wouldn’t really consider to accept to join the Horde (even just for survival) as even with Sylvanas and the Forsaken they had a lot of doubts about joining the Orcs initially…so it’s iffy, maybe the Blood Elves would have just rejoined the Alliance, eventually, but then again the Alliance had made another hostile move against them with the Sentinel Spies in the Ghostlands in TBC, so maybe not…
P.S. nice mog on your Blood Elf Hunter, it’s very nice and fitting looking like Blood Elf Sylvanas, and I guess on the Argent Dawn you could technically RP as she being a Windrunner family member like a cousin of the sister who survived and stayed loyal to Quel’thalas and the Blood Elves - at least she would be the first Windrunner family member to be alive and loyal to her nation for a change! like Zendarin Windrunner who was a cousin of the 3 sisters, but then again he was evil…