I agree with you here. I think that the first of two major points of this issue is as you've stated, the fact that the player basically has no say in what happens as all the ending-choice is made right before said ending. There's no resolution to the problem, there's nothing remotely done for the problem to be fixed. She either dies, after just having felt what it's like to be alive (some will say that it's better than nothing, and I will raise you that it's fucking cruel), or return to the place she would never willingly set her foot in again. Either ending feels like a cop-out of doing anything at all, and what bugs me with it is that even the characters in the group comment about there having to be a way to fix it. Yes, I can already hear someone saying, oh but it's the hope that kills. But that's the thing, we came to love her and throughout the game we wait and wait for more subquests to pop up so we can keep that thread going. But nothing happens! Compared to the shitload of stuff in the journal that we get to follow for the other Origins.
After fixing her heart enough to let her be touched, when talking to Wyll he'll say "We'll get Karlach fixed up, right? There's always a way - there *has* to be".
And then we go on our merry way doing everything and fixing everyone's big and small problems whilst Karlach will randomly say "Damn, it's good to be alive!!" whenever we take control of her. URGH it sits so sour in my mouth!
I'd feel happy about anything that would remotely suggest that we're getting something. I don't even need details, I want to know that they've heard the fanbase, they've read the threads and they see that the epilogue may have added to the existing stuff but didn't really respond to what the community was really looking for with Karlach. And that they've taken that into consideration and will work on something that will give us more options for her story and her ending, that is more hopeful but will require quite a bit of work to implement ingame. I'm ready to give them months, a year, if need be. I'm patient. But I need to know that this isn't in vain and that fanbase voices still matter. Because the rest of the game is pretty fucking stellar, yet this is a real crucial point of interest for me, no matter if we play as her Origin or have her as Tav's companion.