I'm currently playing Fane as an evil manipulator type and a evil resolution to Sebilles quest seems to have been missed. There should be a option to use her Masters (I'm blanking on his name?) song on her mind controlling her to the main character. Either through negotiation with her Master or manipulating her into sharing it with you. A lot of your party members would hate it and you would have to deal with that. It would provide a suitably twisted end to the quest , provide a really interesting change in relationship with one for your party members and settle with Sebille the argument over who should be the next Divinity later in the game.
Basically it seems like a missed narrative opportunity!
