I think that overall Shadowheart has one of the best romances in the game, if you take into account all of the smaller interactions where you can give her gifts and stuff. There are a couple of things I would like added.
First, a little bit more romance-specific dialogue in Act 2. As it is, a lot of the dialogue with her is the same whether you are romancing her or not. I’ve heard that they added romantic banter in Act 2, but had a couple of folks tell me they hadn’t seen anything like that.
It would also be really cool in Act 2 if they could add some kind of interaction with the pair of rings that allow you to cast “gentle warding bond.” Possibly talking about that in relation to the wound on her hand. I don’t think that she needs a whole romance scene in Act 2 personally. It’s a short act and she plays an enormous role in it.
In Act 3, there is one cutscene that I would like to see added more than anything. I would love a camp scene where the romance interest fully reacts to the Dark Urge’s decision-making at
the temple of Bhaal. Especially if Durge rejects Bhaal and is murdered on the spot. The lack of reactivity there is so egregious. And it would be nice not only to have reactivity in dialogue, but also have a camp scene. Especially considering that some of the other origin characters get camp scenes after their quests are concluded.
It would be cool if Shadowheart had a date in the city like Karlach, or something. But, like I said, the thing I’d like to see most of all is under the spoiler tags. I think that Act 3 generally speaking suffers severely from a lack of camp scenes.
Also I want her frickin’ kiss at Grymforge fixed, LOL! It’s been broken for four months! It’s driving me crazy. By the way, I always romance Shadowheart as a cleric of Selune, which makes the romance more narratively satisfying. So that may also enable you to enjoy it more.