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.

2. New options after passing the dryad love test if the romance was discontinued earlier.

Larian said before the release they wanted the romances to feel very real. One of the things that can happen in real life is that sometimes people break up, even end up with someone else, but then they get back together with their initial partner. You can't do that in the game. Once you break up with a companion, there's no way to rekindle the relationship. I think there should be a moment in act 3 where you can change your mind about your previous decisons. The love test is a fun gimmick scene but it could actually become something more and serve this purpose. You can ask any companion to do it with you and you basicly admit then that you have feelings for them. Surely that would make them think and see Tav in a new light, right?
I think it would only work if Tav had heard Astarion's confession, and they had high approval. If you passed the test, he could tell Tav afterwards that although the whole thing had been pretty entertaining, he was wondering about Tav's motivation for doing it with him if they'd previously offered to just be his friend or got involved with someone else. Tav could then pick one of the following options:

a) although at the time they thought it was the right thing to do, they can't stop seeing him as more than a friend, the feelings are still there
b) they still have feelings for him and regret getting involved with someone else
c) they still have feelings for him but it might be better to stick to the current arrangement
c) it was just for fun and the dryad is clueless (a total lie)

If interest in him is expressed then Astarion would ask Tav if what they meant is that they wanted to get back with him. Then the next lines would depend on his approval:
- if exceptional he would say he'd never stopped having feelings as well and that that is what he would like.
- if very good that he'd thought about it a couple of times and he would then ask for a confirmation if Tav was sure about it.
- If good, Tav would have to persuade him to get back together.
- if lower then he would turn Tav down.

If Tav was with someone else, then Astarion would ask them to deal with their current partner before they proceed.

3. Romanced Astarion should have different dialogues when he's dismissed from the party.

It's quite jarring when he's telling Tav 'I thought we had something special' when it's already established that that's the case, even more so after finishing his quest. He should have different lines for Act 2/pre-Cazador and then new lines post-Cazador for spawn and ascendant paths. Maybe he could even have different lines if he'd been dumped at some point.

4. A gifting option for Astarion.

Shadowheart can get a flower and I'd like to give him a small gift too! He has this funny banter with her about missing his calling as a perfumer, so I think a perfume would be a nice present for him.

5. Astarion should totally have a funny comment when distracting disciple Z'rell with lusts for him.

I mean, everyone has a unique comment depending on how we've dealt with the goblins a moment later. It should be the same in this situation, especially if you choose such a hilarious deflection and Astarion is right there and he can hear Z'rell drooling over him.