Just to be clear, I'm talking about the cut epilogue voice lines here and not about Karlach or Illithid in general.
Karlach's lack of story probably comes from her being added so late in the development, hence bad writing and implementation of her quest line. But, honestly I do not know, I've never paid any attention to her, so I do not have anything to go by. As for Wyll, he seems to be even more boring now then he was during the EA.
But in any case, I'm sure you are correct about their development being unfinished, unsatisfying and wanting. But that really doesn't mean that the rest of the epilogue has to suffer. There is literally no mention (I may have missed it of course) in the lines of any origin character by name in romance with another, except Karlach. Are we only supposed to romance her? Is she the only one that gets to be named and the rest gets a generic spouse/partner, I hope not.
Anyway, at this point all we can do is wait until the epilogue is out. We'll see how it turns out, it may be the greatest epilogue ever!
It wouldn't take much work to change it to another name, or a generic term for Tav. The other versions are probably just hidden away in Larian's recycle bin and not thrown on Youtube. I'd much prefer a "show, don't tell" ending for the origin companions though. I'll gladly accept a "tell" ending for all the allies we made throughout the game.
As for Karlach and Wyll, Karlach's problem is still just the lack of agency over her outcome despite the hints being there that you can change it, and Wyll.. he's not even necessarily boring, he just gets way too little. Doesn't he have at least 2 hours (20%) less content than the second lowest companion? It shows, at least.
