I think people are overlooking the Avernus with Tav option. Knowing Zariel in DnD lore, she would accept them both as Champions, and they could fight the Blood War together. While this seems like a cruel fate for Karlach, from her dialogue we can see that she didn't run from Avernus because she was a 'slave', she ran because she was alone, having her chance at love and connections stolen by being stowed away into a realm of heartless Devils (enforced by her Compassionate approval tag). She had respect and titles (Demonbane), Mazora was jealous of how much Zariel favored Karlach over others, and Karlach admits that she enjoyed fighting demons (further enforced by her Violent approval tag).
It would be very in character for Zariel to forget about Karlach's insubordination should she come back to her willingly, and pretty much guaranteed with Tav offering themselves as another Champion (A base level 12 character with direct patronage from a Lesser Deity is extremely powerful, even more so in the Deity's own realm of influence). It is bitter sweet, and horribly resolved in the actual game with no sliders or post credit scene or quick in-game battle against Demons surrounded by friendly Pit Fiends etc., but Tav and Karlach continuing their lives and romance as Champions of Zariel is not a bad fate by any means, and would cure Karlach of the loneliness that ate at her in Avernus. The main issue with this ending is the execution, the Soprano's style fade-to-black with no shown or implied resolution. For all we know Zariel immediately kills Tav and forces them into a soulcoin to punish Karlach (very unlikely, Zariel puts protecting the multiverse from the Abyss above all else) or they arrive in Avernus and immediately run for their lives being hunted like rabbits. The resolution isn't there, their fates are left to interpretation which in other stories can work, but doesn't work here.
Now I'm not saying that she shouldn't have a more difficult resolution to her quest that sees her live happily in Fayrun alone or with a romanced Tav, she should. Returning to Avernus with Tav should be an outcome from a quest-fail state combined with a successful romance, not the best outcome. However it's the best outcome we get, and knowing DnD lore, their fate would not be a terrible one. They get to have their fairytale romance in the downtime between fighting in the Blood War. We are just not shown this, and Larian failed to resolve the storyline they implemented with poor execution rather than poor ideas.
Pretty interesting point of view. I'm glad someone talk about it from Zariel perspective and not like "IT'S THE TRAGIC CHARACTER THAT'S ALWAYS LIKE THAT IN ALL SCENARIOS STOP CRYING BECAUSE U DON'T HAVE A GOOD END", like it was a serious argument regarding her script weakness after Act 2 and the "impossible becomes possible" aspect of the lore. I assume that's a much "good" end if she goes into Avernus by her will rather than being forced to do it.
To me all the options should appears in the game. Because all ends should be dependent of how the player plays, regarding player success and player failure. I'm not into a railroad end that extreme in the Forgotten Realms where you can barely do anything like cheating death or killing gods. I find it incoherent at some point you can't have opportunity to repair a simple old mechanism, even more when the game introduce you to literally the inventors of her mechanism.
A bad end where she become a mindflayer (if you convince her to do it). A dead end where she... dies (if she's alone). A bad end where she have to go back to Avernus against her willing (maybe friendship with PC, Wyll is not the Sword of Avernus AND she doesn't have her heart fixed). A bitter sweet end where she's the Fury of the Avernus (Wyll is the sword of Avernus/romance with PC, AND she doesn't have her heart fixed). A good end where she can stay in Fayrun (her heart is fixed). A good end where she can stay in Fayrun + PC romance (her heart is fixed + romance with PC)
All the ingredients are already in the game to have all those ends doable without adding hundreds of subquests or new characters by dozens. 2 or 3 more steps like talking to the gondians and provide him required ingredients, 5-6 new dialogues, 2 new cinematics and 2 new end cinematics and her story is completed.
