Again, would prefer a fix quest for Karlach in the base game. But coming back from Avernus and chilling with friends would be very welcome.
Pretty much how I feel. A more thorough fix to the way her story is written would be ideal for me, but I think a more substantial epilogue like this info hints at might be the best we get. I'm not really familiar with how Larian works since BG3 is my first game from them, but it sort of seems like the level of rewrites/extra content specifically for Karlach that so many people around here want is rather well beyond the scope of what you'd expect pretty much any developer to do, especially post-launch.
You may have heard about their rewrites for Beast (and Fane) from the Divinity series. Here's a video mentioning the changes made to Beast's story. Essentially, the main complaint they received about him was that his story felt incomplete so they gave his questline more substance and polish so that it feels complete. Much like Karlach, he became an origin character much later in development and similarly suffered for it.
Make of that what you will. However, keep in mind that the complaints for Karlach's story are different than the ones for Beast's. Plus, he was nowhere near as popular as she is, so you shouldn't expect them to repeat what they did for Beast and simply pad out her existing questline.
I believe they will do right by us and give us what we want, even if it might not be exactly in the way we imagined.
