Obviously it's no secret that the general attitude about Karlachs endings are largely negative and even more negative about the flow of her questline.
I think adding a few more endings would give her story more depth while still giving the chance for a bitter sweet ending.
1) House of Hope: it never really made sense that Karlach absolutely rejected the idea of staying in the HoH but can be convinced in the last 30 seconds of the game to just randomly spawn into Avernus.
This ending should be unlocked after taking Karlach to the HoH, saving Hope and defeating Raphael.
2) Fix Karlach: this is the obvious one but should come with harsh conditions.
Dammon must survive to Act 3.
Baucus must survive to Act 3.
Baucus must become leader of the Iron hand Gnomes and negotiate cooperation between them and the Gondians.
ALL Gondians must survive the Factory and Iron Throne
Gortash must be defeated.
After these conditions are met, the option to gather Enhanced Infernal Iron and a McGuffin from the HoH and forge a coolant system for Karlach should become available.
3) Heart Transplant: This is the bittersweet ending I would ACTUALLY be ok with.
After all options are exhausted, Mizora will approach you with a proposition. Give Karlach YOUR heart and take her engine and her place in the Blood Wars as Zariels assassin.
This option should only be available if you are romancing her. No other conditions required.
