Hm, I'd like a mixture of developing a relationship by actually interacting with the characters we can romance and having them frequently be a part of our quests so we can make "more informed" decisions on who we like more, etc. Ex. Triss & Yen are both part of a quest, both offer different solutions and we can make a choice between one or the other (really, really generic example).
We should definitely see the impact of our relationship with our interest(s) in our quests as well imo. Ex. If we like Yen and the game registers that because of decisions we made earlier in the game, they should have additional or unique dialogue or choices we can make within a quest.
But what we get is what we'll get.
Edit - Added examples.