I also wanted to build a little bit of your idea some pages back on going back to hell as part of her quest.
I remember you mentioning about going to Avernus and finding a mechanic, getting her into the operating room and having to defend it like Halsins portal. What if there was also a use for soul coins in the situation?
I don't know about the rest of the players, but I never use soul coins, ever. I don't even know what kind of power boost they give Karlach at this point. But they are a bargaining chip, especially with devils. Considering it would be a whole lot of hell's denizens descending on you what if you had a chance to pay them off? Have an option to negotiate with an imp or some mediator to give X soul coins to the equivalent of guard captains or squad leaders, so that when the call to attack you goes out, they are conveniently busy. That way we get a use out of soil coins beyond powering up Karlach, and if you're been 'good' via not using up their souls, you make it easier to save her life.
