Realistic thinking: Not dead could just mean she ends up imprisoned… Though personally my bet is on her being influenced by old gods/void lords and/or having some secret plan against them. So the whole thing will end with either her being ‘redeemed’ or we find out she was right all along, just her methods were not the best, and we should all feel bad for calling the writers-- I mean, calling her dumb.
Optimistic thinking: Sylvanas time travels back to just before the Forsaken joined the Horde, taking over for her past self and convincing Thrall NOT to let the zombies in. Forsaken are removed from the Horde and become a neutral antagonist faction similar to Naga; the Horde regain their glorious original aesthetic and feel, and all Forsaken players get a free race change. Blood Elves can stay, at least they got the red color scheme down.