It will be Sylvanas.
Suprised you didn't mention Chromatus as part of the Twilight dragonflight threat. Imprisoned and still a huge undealt with threat.
Alternate theory:Maybe it will be Turalyon. His Naaru sugar-momma got shattered and his li'l yellow eyeballs stopped glowing yellow, so he's ripe for corruption. Now he's planted his backside on the throne in Stormwind (rather than take a seat nearby,) and when spoken to, will sometimes say, "We didn't come this far to falter now."Hmm... where have we heard that multiple times recently?