May be this will follow the Kerrigan plot line, where the strong female main character is transformed into the enemy. There could be a power struggle between the off springs of the 7 Evils (or remaining lords of sins) just like the cerebrates competed with Kerrigan for the control of the swarm. Leah could discover ways to make herself more powerful, perhaps seeking to find the Black Soul Stone that fell from Heaven from the ending cinematic, and make herself the Prime Evil.
And then, may be we will gather pieces of an ancinet Xel'Naga... I mean Nephalem artifact and invade the Burning Hells to turn Leah back to human... Ok, I'm getting carried away XD