There is no actual romance path with Kas. She's just a shell. A McGuffin to advance the story.
Hear me out a sec. What we have in the game now is built on sex, lies, and half truths which are no better than a lie. There is no real open communication with her, nor the ability to just sit down and talk about how the PC is, feels, and thinks. We don't ask her how her day is going, we don't really actually get to know her past from her. From the PC's point, she is an entirely unknown variable that they're either somewhat or diametrically opposed to. We're never given the option in the encounters with her to get to learn about even an inkling of this without submitting to someone who wants to use and abuse us. The rest of the information we glean is questionable at best, outright wrong at worst.
What I fear the most is that her daughters, despite still having their souls, are going to be EXACTLY the same. Even the one who willingly comes to you to get away from her own mother's plans. Even if it's only for a little while.
I feel like we're given two options. Kill her, or submit to her and her plans. I also feel like we're being told that the only correct path is to kill her, submit to her, or die. I don't want to kill someone before giving them the chance to understand my point of view. It all feels incredibly one sided, like I'm being told that I'm wrong for wanting something else. Yet a real relationship is one that is founded on understanding, communication, trust, and just a bit of compromise. Sure, the PC may never be able to get her to become a pure individual, but she should understand that she shouldn't be forcing her own views on her partner. (Or others, for that matter.)
With all the other partners in all the other games, you're given a chance to get to know them from their mouths, get a feel of what they want and like. The nature of the world might lead to a lot of spontaneous sex, sure, but you still get to know them.
Kas? You're expected to love her instantly. To want to love her instantly, but she's just a hollow shell right now. She has no real substance. She doesn't even object to you killing things that might have very well come from her own womb nor does she stop you from messing around with her plans. You have no interaction with her family, and to top it ALL off, she wasn't even SUPPOSED to come through that portal. Nor was she supposed to bring an army though. She's being chased by what is implied to be a void god or at least a powerful void being of lesser standing that easily destroys her entire army. Which we can thus assume is the same result if the portal is closed with her army en-route.
Believe me, I want to love Kas, but I just can't. It's not a matter of compromise, it's a matter of it just feeling utterly wrong from a PC's point of view. Now, it's not so wrong with a "corrupt" PC, but it still feels utterly empty with absolutely no road for redemption in sight.
Of course, that's just how I feel about her right now. All of my opinions could change drastically with the next major story patch with my fears becoming nothing more than paranoid delusions. Yet, with how rail roaded into one side or the other we are, I really don't think this is going to change one bit.