More than likely Sylvannas will be the Tirion who actually ends that fight and proves that she was the hero all along… just like Illidan.
I don’t think she’ll be a raid boss this expansion, but she won’t be Warchief anymore once she’s exposed as just playing the Horde to serve her selfish interests.
She’s going to become significantly more powerful by the end of the expansion… and at a future point the Horde and Alliance will unite against her (World Ending Threat # 17792), and will finally get demoted to loot piñata status.