Nah, changing her quest to not be about her engine is not happening nor should it, it's too tied into the game and it is the basis of her character's entire conflict. Having an Avernus DLC where we fix her engine is also very unlikely as it would require a massive amount of work just so they can fix one character.
The simplest way Larian can fix her ending is by giving us the option to talk to Zanner Toobin about her engine after we've saved the gondians and done the Steel Watch foundry quest and killed Gortash. Have Zanner tell us that he and his engineers can convert/replace her engine into the plane-stable version they've designed for the Steel Watch but he will need X amount of Enriched Infernal Iron to do it. Then go to him and have him apply this final fix and voila, Karlach's questline is complete.
The work involved for Larian in this resolution is pretty minimal. A few extra dialogue options for talking to Toobin and for talking to Karlach after her engine is fixed. A couple of extra cutscenes, one where Toobin fixes her engine and she is elated and relieved, and one for the ending where instead of the forced tragedy she is happy, grateful and looking forward to her future. Remove Karlach's doomer dialogue options after she is fixed. Finally, if on this route, disable the dialogue option which leads to Karlach volunteering to become a mindflayer - she obviously would not want to do that anymore now that her engine is properly fixed.
This turn of events preserves the whole dead-woman-walking idea right up until almost the end, allows for Karlach's post-killing-Gortash rant to still happen (and it should because that's one of the best moments in the game), allows us to use one of the very obvious in-universe solutions to Karlach's problem that anyone with two brain cells to rub together can see, gives a purpose to the Enriched Infernal Iron that we all know was meant to be used for exactly this, the bad endings are still there and there are plenty of ways you could mess up and get them, AND it requires minimal work from Larian.
This is likely the most we can expect from larian, yes. It would still leave karlachs questline to be rather dull and not give her big character moments/her own dungeon area for us to explore like the other companions get, which is disappointing. Still, id rather take a small patch that let us fix her engine rather than nothing i guess
