If we go with Shadowheart is a Selunite sleeper agent theory then we could have a choice scenario.
The first night after entering Baldurs Gate, Shadowheart is missing from "camp".
We can track her to a temple of Selune where she is the focus of an occult ritual. The Selunites try to persude us that Shadowheart is one of theirs, a volunteer who infiltrated a Sharran cell to get the "weapon" into Selunite hands. Shadowheart begs us for help to escape.
We can choose to free Shadowheart (and keep her as a Shar/Trickery Cleric) or allow the Selunites to continue (and have her rejoin the party as a Selune/Light or Life Cleric).
They could add to this with follow-up quests.
If SH is still Sharran, perhaps we discover evidence in the temple that the Selunites were telling the truth. If we give the evidence to SH it may soften her personality a bit, make her less sure of Shar. Or we could withold the evidence to keep her strong but evil.
If Sh is Selunite, we could find evidence that the temple of Selune hadn't been fully honest with her, and their plans for the "weapon" are a bit worrying. Again withold or expose.
I could see SH becoming this games Viconia, especially if taking certain choices could cause her to abandon both Shar and Selune, and pledge to a neutral deity instead (or even to the Absolute, if you really mess her up!).
