I just finished my first playthrough, with unascended Astarion, and I have to agree with what everyone in this thread has been saying. I feel like his romance arc peaked at the graveyard scene, and everything after that was either nonexistent (to be fair all romances are like that though) or just so watered down in comparison. Why don't we have one last sweet scene once it's time to go and fight the final battle? All we get is "this could be our last kiss" and then it's the most chaste kiss I've ever seen in my life. Then there's the awful scene at the docks which may have been fitting for a playthrough of someone who ignored Astarion the entire game, but not for someone who took the time to get to know him and complete his personal quest...and definitely not for someone who romanced him.
But honestly my biggest gripe is with the final scene you get with him just before the credits.
Aside from how weirdly abrupt it starts (we just saw him run away in terror from the sun and now we're...where, exactly?), it just feels so loveless! Where's the emotion? Why can't we hug him or kiss him? Why is Tav standing 5 feet away from him with their arms crossed? If you changed only one or two lines in the entire conversation, you wouldn't even know that this was a romance scene, it could have been a scene between two platonic friends. I'm sorry, but Larian...what were you thinking? This the very last time we're ever going to see this character that we've grown so attached to over the past 80+ hours! It shouldn't feel so empty and lifeless. I don't care that there's no sex. I don't even mind the fact that there's no magical last-minute asspull cure for him, because you do get the option to bring up finding one and I'm content to let my imagination do the rest. But I really wish it didn't leave me with the feeling of "That's it?"
I hope that they can improve the endings to some extent, either in future patches or even a definitive edition of some kind. It's an amazing game and an amazing romance otherwise.
Also maybe it's just me, but I reeeally wish we had more opportunities to tell him he can feed on us again if he wants to. I didn't give him the option to do it a second time because he and Tav are still basically strangers at that point in the game, but that doesn't mean I wouldn't want to bring it up again later after romancing him. Ain't no way I'm going to let him survive off of rats for the rest of his life.
