I wouldn't mind if Blizzard pulled a cheap shot on the LotV storyline and crushed Amon's plans halfway through the campaign. Then the other half could focus on the other races again, fighting for dominance in the sector. For example, whoever kills Amon can potentially wield more power than anyone could have ever believed possible. Thats the only realistic scenario I can see in which the Protoss, Zerg, and Terran start fighting for control again. Everyone wants that power, regardless of the reasons for it. Zerg want power, Terran want control, and the Protoss could simply wish to seal it away. I wouldn't mind to see a return of the UED either tbh.