Open for sure. With the worldstone gone, Lilith Azmodan and Belial all still roaming, and the fate of a soulstone still unknown there are so many possibilitys!
Unless you count ubers of course, lilith died. But evil cannot be slain forever outside of hell. Azmodan and Belial are not in the mortal plain, but since the destruction of the worldstone evil can cross freely into the mortal realm from the burning hells.
Is it possible that Azmodan and or Belial could steal Baal's soulstone in order to drain its energies to fuel a war against heaven? Does a soulstone hold ambient energy? Can Azmodan and Belial work together long enough to carry out a full scale war?
for those who havnt seen it before here it goes...
Tyrael was peirced by a worldstone shard and baals wanderig soul entered tyraels body via the shard using it as a sort of makeshift soulstone thus creating an evil with all the powers of heaven.
Baal with all his new power decides to ressurect his two soulstone-smashed brothers back to life as zombies under his control thus the zombie-like Diablo on the alleged diablo 3 box cover
Baal is attacked by hadriel and several other angels when he visits diablo's resting place in hopes to zombifie him but Baal escaped and still managed to manipulate his brothers corpse.
Baal and Diablo proceed to attack travincal while the new council, some strong mages, and a few angels await. Baal with his new powers and zombie brother smashed the holy defence and ressurected Mephisto.
Meanwhile in Hell: Belial and Azmodan grow impatient and take control of hell in the prime evils' absence and start infusing the demons with the defiled shards of the worldstone giving them a boost in growth, power, and demonic aura.
Belial betrays Azmodan and dispatches him with one blow. Belial consumes Azmodans soul with help from a corrupt Horadrim. He becomes a true force to be reconed with.
Baal returns to see his former demons under belials reign outside the seventh gate of hell... The stage is set...