I've been thinking some more and have come up with new suggestions for the romance. I'll start with the one that would need the most work, down to the least complicated one.
1. An extra scene in act 3 after finishing Astarion's quest, basicly a date scene. - If Tav is Astarion's consort he could take them out into the night to teach them more about vampirism and how to hunt. Also, he could show off his new powers without prying eyes nearby. - If Astarion is a spawn he could take Tav out into the city and show them his favourite and/or secret spots where he used to hang out.
I'm almost sure we need Upper City for this. Astarion even has a comment about upper city. I am sure this is his real location, not the lower one.
