I would like to agree with Nyloths first post. But disagree with another request.
I speak for myself: I am role-playing a good character and the positive meant (?) answers on Astarion's question "So, tell me what you desire. What can I do for my dearest pet?" are more than odd to me (or let's say for my character). 1. "I want to be a vampire, like you." (My good Character never thought or wished to become an ascended Vampire, neither I, this is so out of the blue and out of character) 2. "I want you. I want your body." (After the act 2 emotional romance scene, which good and caring Tav would ever answer "I want your body?", even if deep inside they would like to sleep with A. This is so out of character, too, for my good Tav.
So I also would like to have a positive answer (for a good Tav) like: "Being with you" (instead of "I want your body".) And everything would be fine. Astarion then could laugh and say the same thing like to the 2. answer, just : Of course, my sweet. "But don't you want more... etc. One more bite...etc. ..To be mine? Forever?"
A. Astarion's ending is fine for me. My Tav (when romancing him) being forced to be his spawn is totally fine for me and I would never ever want this to be changed or having an option to stay mortal in this scene. Like it is now, it's a great power-dynamic story. So yes in this, I do not agree with the request of rewriting the whole scene for having the option not to become his spawn. I don't want this great story to be destroyed. This would change A. Astarion. And he is, how he is, he wants to control. I don't want him less evil. For the "equal" thing we have the other ending. So this is my opinion.
I don't know why the discussion had to lead to this odd "Astarion becomes Cazador. Noone chooses this ending." (?), as if this is a fact. Wrong. I chose it. And I will choose it again. Also equating Astarion with Cazador doesn't do him justice.
"I would, thank God, watch the universe perish without shedding a tear."