Unlike quite a few here, I really like the vampire gaining their souls for redemption story arcs but the Drusilla is actually the only one I DO NOT want to see regain her soul. Angel tortured her and not just physically. Her killed her family, told her she was evil and just drove her insane and waited for her to become a nun before making her eternal. Drusilla is too far gone for a soul to have any 'good' effect on her, IMHO, a soul would drive Drusilla off the deep end. She would ultimately go on a killing spree before killing herself or finally being killed by a slayer or Angel or most likely Spike. It would make a for a great last run for her character but TBH, I'd rather her just be out there doing evil without a soul and never return to the series.
I couldn't see her going on a killing spree if her soul was restored. I think she'd take her own life out of the grief of reliving all the bad things she's done. The pain would be too much to bear. Why would she kill if she felt bad about her actions?
She wouldn't feel bad about her actions is my point. She is certifiably insane. Gaining her soul would just cloud her mind with more thoughts of pain and she would just want the world to bleed.
The very definition of insanity is the inability to think about the consequences of her actions. Drusilla wouldn't care about what she's done and even if she does, not in the 'I want to redeem myself way', it would be the 'I want to make the world pay for what it did to me way'.
But even if she did feel the weight of her actions, it would only drive her further into the psychosis. She wouldn't be able to recover from it. One way or the other, a soul would only end Drusilla.