Yes, there's content for the other companions, but certain events - like the one you had with Karlach - cut off subsequent scenes. Non-companions don't get any content.
there is SO MUCH in-game time devoted to helping all the non-companions, and they really felt like part of my camp. And then at the end they get ...... NOTHING. Simply stunning. So there's basically no point whatsoever doing anything with any of them. Since I spent MOST of act 3 at max level, it'd actually be better NOT to help any of them at all. Nothing you do with them matters at all, and they have no ending segment at all, so having less xp all throughout act 3 would be more fun. It wasn't fun to spend the entire act 3 with zero character progression. What a disaster.
The endings are awful.
Here's how my first ending went as it was somewhat similar to yours and this was the first "dagger in the heart" moment for me with this game:
After a lackluster Act 3 with little to no reactions from companions, the thought was "well this is rushed, but maybe they stick the landing with the ending."
Couldn't have been more wrong.
I went with a custom Tav. She was a drow sorcerer that romanced Astarion.
I LOVE Astarion. He's the best, most well-written character in any game I've played. I know you played his origin but I highly reccomend trying him as a regular companion because you won't get anyone more cheeky, sassy, complex, emotional, or with depth of character.
I was completely enthralled and excited to see what ending they had cooked up for us.
I convinced him not to ascend, like you did, and we got to the docks after beating the elder brain.
I was waiting for an ending moment with him. Something poignant and meaningful and some sort of end romance scene to cap off the experience.
No. Nothing. He burns up, runs off, and my character just sits there dumbfounded while one of the other companions, in my case Wyll, says a snarky and annoying comment about him "remaining in the shadows" or some shit.
So basically, I just watched the love of life who redeemed himself in every way to the others at this point, stumble and fall in a comical fashion as he burns up, and my Tav, what, doesn't care?? Doesn't run after him?? Doesn't even wrap a cloak around him?? Nothing?!
Not only that but, while I was able to convince Karlach to go to Avernus, her ending was only one who got any attention. I wanted Gale to get the Crown and his ending was just "Okay. I'll do that. I'll become a god." And then he walks off...
Then, Shadowheart is there, my second favorite character, and I was interested in what she was going to say. Guess what? Same as you. Nothing. She says literally nothing.
In addition, I learn nothing else about any of the other companions, learn nothing about my impact on anything, no final cutscene or party. It was absolutely the most unsatisfying ending to any game I've played.
The only solace was, I did get a final conversation with Astarion (you know, after finding out through no effort on my character's part that he didn't, ya know, DIE) but even then, you and Astarion stand ice cold, 10 feet apart, show absolutely no affection while he basically reaffirms "Do you still like me??? You sure??" And that was it.
Complete and utter bullshit at every turn. I hate that you had to enter the endings without a forewarning. It really is, as you stated, a slap in the face.
Man I absolutely loathe how this is all turning out, but at least it makes me feel a bit better that I'm not the only one. I'm sorry that you had that experience, same as I did.
I think the problem you have is the fact that Astarion, even if played as the player character, will always run off to the shadows (if you didn't ascend, and it sounds like you didn't) before Karlach explodes. So there is no chance for Astarion to say his final goodbyes or to convince her to go to Avernus. What is at fault in this scenario is the timing, it always happens like that, in both versions. If played as Astarion and if played as anyone else.
But you should see this cutscene as his proper ending: ... Which I am betting you didn't get to see either (it's either this or going to Avernus with Karlach) because patch 4 messed up a lot of ending epilogue sequences.
The only way for Larian to fix this shit is to rearrange the dock cutscene sequences, so Astarion doesn't fucking burn at all (it is sunset/evening after all) or he runs off AFTER the big Karlach choice.
No I definitely didn't see any ending whatsoever for Astarion, or Karlach, or Wyll, or Shadowheart. Only got a boring useless "ending" for Gale and a good ending for Laezel. Which is hilarious for me because I hated Laezel and never used her in my party ever. LOL so she's the ONLY one that got anything. Of course.
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I have been gaming for going on 35 years now, must've played thousands of them over the years. This is BY FAR AND AWAY the absolute worst ending to any game I've ever seen. There's not even a close second place. I guess it's difficult to even have a second place, because here, there was no ending at all for almost everyone. Just totally missing.