I did cover the Avernus ending (somewhat extensively, with options) in my original post. Fact of the matter is for the entire game (until that end), Karlach is adamant in not only her refusal to return to Avernus, but also returning to Zariel's servitude as the 'Fury of Avernus'. That simply wouldn't be any shade of positive ending for the character. Best case scenario, Larian forges a good ending from the pieces left on the table in Act 3, and allows us to fix her heart. But second to that, the House of Hope, Doomslaying through Zariel's forces and demons in Avernus, some kind of positive epilogue. But certainly not serving Zariel in any capacity.
They wouldn't last long having to contend with the Abyss and Zariel's forces, Karlach would lose her patronage from Zariel and would not have the powers she required in Avernus to survive. The positivity of this ending would be having Tav at her side, and I believe that would bring her happiness whether she served Zariel or not. Of all the Archdevils, Zariel is the most motivated and and has little care for personal slights if a resolution would aid her in the Blood War. She would likely even allow them permanent residence in the House of Hope and to live their lives unimpeded and with dual patronage should they aid in her efforts against the Abyss. Don't get me wrong, I despise what Zariel did to Karlach, but there is no questioning that she is the most fair and least deceitful of the Archdevils, due to her dormant Celestial nature.
If Larion cares enough to resolve Karlach's questline with an ending that is remotely satisfying, it is most likely to be something involving Zariel and Avernus, and unfortunately two level 12 characters stand absolutely no chance in Avernus without allegiances and patronage. Even at level 20 they would be living on borrowed time. Karlach despises Zariel because of what she took from her, not due to her treatment over the 10 years of her servitude. There can be no happy endings in the Nine Hells, the nature of those planes quite literally forbids it. So Larian can either write a completely new outcome to her storyline that involves her deserved fairytail ending in Faerun or she compromises in Avernus and plays the best hand she's got, with the person she loves.
Step 1: Be in Act 3, learn from the Steel Watcher that Karlach's heart is an early version of theirs. Step 2: Rescue Zanner's daughter, tell him. Step 3: Clear the Steel Watch Foundry Step 4: Tell Zanner about your issue Step 5: Compassionate because you saved his daughter, he builds a final heart with Dammon for Karlach.
3/5 of these are already ingame and provide a much more satisfying ending than returning to Zariel's service, which is something Karlach clearly doesn't want to do. She wants to live, on the material plane, and the pieces are right there.
I understand your point, and the lore (I even played a D2A game where we redeemed Zariel). But if they were to expand on the Avernus ending, it would go entirely against Karlach's character to actively seek servitude again, even as a Champion. Allying with a different evil to defeat Zariel toward the end of leaving Avernus? Very possible. Utilizing the House of Hope? Very possible. But there's simply no logic to saying that Karlach (and Tav by extension) should return to the slaver who she would rather die than return to service to. The reason she returns is to live /with Tav/ not because Avernus suddenly became the better option than death, but because Tav (And Wyll, who clearly states he'll protect her from Zariel) is with her.
