Adding to the Melissan thing, I also think there should be some way to make her "give up" as in, turn her to the light side again, lol.
The Solar that is babysitting you only comes when she feels like it. But what if you could somehow, when you first meet, if you have a good alignment and high wisdom you could come to a deal with her to be able to summon her yourself once only? Assuming,the PC isn't a total newb, he should be able to somehow realize that Melissan isn't what she seems and summon the Solar to make some things clear and ask whether or not he can help Melissan.
I know it's a bit of a stretch, but nobody is born evil unless they have a genetic condition(supergenious criminals). And if you, who has had the greatest part of the essence can be lawful good, why couldn't she?
And if Sarevok and Irenicus can be redeemed, why not her? I have no plan yet a to what would come out of this, and it should happen at the very end so as to not destroy the whole plot.
I personally hated Melissan, but not as a person, as a Character. She was too.............flat. A character is only deep when it has both good and bad sides. I know she was a priestess of Bhaal and they're supposed to be evil. But look at Nyalee, she honestly loved Yaga Shura, although he was 50 times her size lol. I hated that you had no choice but to kill her.
And Imix should just die. One less bother for his Fire Elemental Prince rivals lol. There could be a planar travel when another Elemental Prince asks you to kill Imix for him(Lol, I know I'm sounding crazy again hehe). To justify this, you could somehow use the elemental scepters in the Demon level of Watcher's keep to have the portal create a doorway to another plane instead of the next level of the dungeon where you meet Dun'Kel, another elemental rpince that is supposedly either bent on killing Imix or Supposed to do it. He may be forbidden from traveling to Toril but would be willing to hire you to do it. In exchange he could give you an item that would capture Imix's essence once he dies and that would be a new Ioun stone, +50% fire resistance lol. It would be justified because the mages on that level had been crazy and the Fire obsessed guy may have come into contact with another plane in order to gain the advantage. Yeah, another crazy idea but meh
And I also felt the bhaalspawn dorkade(The bhaalspawn party in the Fire Giant Mountains that you kill(Chinchilla the rabbit and the xvart and whatever else they were) should have been more expanded upon. Like, WTF Bhaal had unprotected sexual intercourse with a female specimen of the Rabbit Species? I'd really like to see Imoen's reaction to that

Maybe they could be the ones to infiltrate Saradush with you and strike at Yaga Shura-s forces from behind, thus making your life easier when you finally face him. And Chinchilla has a hell of a running speed lol. Best escape artist you could ever dream of
And concerning Winning Abazigal to your side: You could have the option to transform into the slayer and impress him. I imagine that form should give out an incredible amount of killing intent. I mean Bodhi was scared Sh*tless lol(Although she was evasive after that hehe).And I know there's a mod that upgrades your transformation too(The normal Slayer? Yeah right...It's incredibly weak , 100 hit points? lol)
Another thought... The jumper. The teleporting Bhaalspawn. If it were possible to cast a sort of tracking spell or link spell to him one could actually discover the secret to his ability(And somehow gain it). Yesh, that would probably be too much, but I'm just throwing that out there.
And maybe one could at least save Volo for future reference.
Well, that's all for now
//Sebastian