There have been plenty of problems. In this game the "Warlords" get killed off really fast. There ideally should have been a patch for each of them where they all become the new threat. Also seeing as Garrosh was the main threat in this expansion, killing him off almost immediately doesn't feel rewarding, mostly because he dies in a cinematic as well.

Kargath may have lost in the arena, but that was only a set back. The Shattered Hand operates on the fringes of society anyway and could have plans to infiltrate and sabotage the efforts of the Horde and Alliance.

Catching Garrosh should have been epic. He nearly brought Azeroth to its knees and should have been brought to justice by a tenuous agreement between Thrall and Jaina. Thrall to redeem himself for letting Garrosh lose upon the world and Jaina for vengeance. Despite being friends in the past, this event wouldn't really heal the wounds between them.

Ner'zhul's clan is about raising and communing with the dead, he could easily come back in a future patch, plus his right hand Den'Targ has yet to be seen. After being killed once, he will return changed and once again lead the horde as he did in Beyond the Dark Portal. You could also have him reanimate Garrosh, the champion who rose to prominence fighting against the Lich King, and Maraad, a symbol of the light, who will be nothing more than Ner'zhul's puppets.

Cho'Gall could once again attempt to serve the old gods and he did in cataclysm. The Twilight Hammer would grow to power in Draenor and attempt to do what Deathwing failed to do in Azeroth.

Grom could be the final boss as he should have been. Being cornered by the failures of the Iron Horde and enraged by the loss of Garrosh, he would drink the demon blood. He will reopen the portal, this time with a willing Gul'Dan. His horde would have both the advancements made by the Iron Horde but also the backing of the Burning Legion.