Having FINALLY finished the game and gotten the Patch 5 epilogue, I am extremely confident they're going to do some kind of DLC to rectify the Karlach situation. I shall list my thinking below:
-A common point I see raised about them not doing a Karlach DLC is the simple problem that for a lot of people's playthroughs, she's dead. Consider though, there are a lot of hints in the epilogue that there's a DLC coming whether Karlach's in your game or not. So if there is a post epilogue story, Karlach can just well, not be in it.
-We of course have the discussion with Karlach about the Forge and her getting a new engine, and since others have already discussed it I won't rehash it here.
-Almost every conversation in the epilogue ends with some sort of implication that the story isn't over, from Astarion's "See you later, darling" to Wither's "Thou mayst yet be needed." It'd be a strange choice to include such blatant hints and then have the story, in fact, end there. In fact if your character dies at the end, Withers shows up personally and literally says "Your story does not end here."
-Jergal's mission isn't done yet. If his new assignment as others have theorized is to clean up all his god-related messes, then one glaring mistake still remains: Cyric. If the hypothetical DLC involves taking down Cyric, then that would solve the problem of having a Karlach related DLC when everyone doesn't have Karlach.
-The idea that Cyric is the next adversary for the party to face is further reinforced by a note discussing "a black sun on a white sky" (Cyric's symbol) and the appearance of the musical entertainment. The bard playing music at the party is the now defunct god of music, who was screwed over by Cyric and naturally wants revenge. And, wouldn't you know, he's level 12, same as the rest of the party. This would check off another major player complaint: the lack of a bard companion.
-We also have the appearance of the Strange Ox in the main game, who mentions Cyric by name AND is some sort of fiend, creating a (rather loose) tie between Cyric and Avernus.
-We also have the confirmed cut content in the files of there at one point being a roughly Underdark sized Avernus map, complete with enemies and terrain assets that we then only saw for the opening and the vistas at the House of Hope.
-Outside of game, we have several developments pointing to Larian working on SOMETHING big involving BG3. We have the twitch interview with one of the main leads who implied that even though he can't talk about it, there might be something DLC related in the works. We also have the now deleted tweet from the company's PR manager saying that they're working on something big for BG3 and they can't talk about it yet. We also have the very public tweet from multiple sources saying that Larian is considering doing a DLC and saying that they're not done with the game yet. For a company as tight-lipped about what they're working on as Larian is, these are some pretty big hints.
-I think it's pretty clear that Swen and company wasn't expecting Karlach's ending to gain such universal vitriol, considering not even a month after release they release a new epilogue cutscene with her and friends storming Avernus, AND the Patch 5 epilogue giving us some rather unambiguous hope about her and Tav's situation.
-Swen may like tragic stories, but he's not an idiot. He knows that digging his heels in about a fan-favorite character getting such a shitty ending is a bad look, and why do that in the name of "artistic vision" when you can make a DLC and make fat stacks of cash. After all, everyone loves fat stacks of cash.
In conclusion, I think the evidence points to, if not a Karlach DLC, some sort of DLC. This is a perfect opportunity to wrap up Karlach's story without making the DLC exclusively about her and justifying the cost of its existence. Thank you for coming to my theorycrafting TED talk.
