Copying my last feedback, will probably be my last for a while since I put in 450 hours in BG3.
The near-unanimous consensus on Karlach is that her story feels incomplete in Act 3. She is the only companion (to my knowledge) whose ending does not change regardless of your previous choices in the story. The choices are 'death' or 'Avernus' and you make them in the last 30 seconds of the game. Very unsatisfying. It SEEMS like, from all the clues, that a new ending to fix her heart ought to be added.. And don't make it easy. Make the fix ending only available if Dammon, Barcus Wroot and all the Gondians survive deep into Act 3. Make it so you have to broker a deal with Gortash, potentially putting your kidnapped companion at risk. Make it difficult and rewarding but give the people what we want!
Plus, as Karlach is rather a (chaotic) good aligned character, having all the helpful people saved and kept alive in your game, i.e. playing on a good alignment, can be an intelligent way to challenge the player "if you want to play a good samaritan then prove it and you will be rewarded".
And as you only know at the really end of the game that all those people are the only ones that can help Karlach, it should be a real "difficult" mission to save her. You are not even aware about all these prerequesites during act 1 and 2. You only understand during Act 3 if your choices/actions in previous acts have a positif or a negative impact for having a good end on Karlach quest. Karlach's quest is not the most exciting quest, and I'm not sure it can be a "better" entertainment than "gimme some infernal metal" during act 1&2 tbh. But adding some butterfly effect from other totally not-related quests can add a lot of depth to Karlach's personal quest, where success depends not on which boss you have to beat, but on who you have to save.
In that way, everyone can be satisfied, the players that want a coherent good end, those who swear by the adage "nOt EvErYoNe CaN hAvE a GoOd EnD", and Larian who don't have to touch anything before Act 3 except putting triggers on some quests to activate the final new Karlach's quest+cinematic, which would be the only thing to develop.
