I don't really have anything new to add to the discussion, but I'd still like to add my voice to the pile of the unsatisfied. I recently finished my first playthrough and was thoroughly dissatisfied with Karlach's ending. The whole thing has left such a bad taste in my mouth that I went from planning to immediately start a second playthrough to just not wanting to touch the game again at all.
Karlach was by far my favourite character and you did her dirty - please add some way to fix her heart so I can play the game again without dreading the ending, Larian.