Liara (from Mass Effect) and Morrigan (from Dragon Age), and most other companions in Bioware games are important characters to the plot, and so the player character could react to the unfolding plot when it affects a companion. That said, the romance was still optional, and that limited the implications.
I feel that romance works better in games that are more linear. When I played Final Fantasy VIII, I really enjoyed watching the romance unfold between Squall and Rinoa. They both influenced each other as characters, and Squall didn't even realise how much she meant to him until she was abducted and he realised he wanted to save her more than anything. Rinoa essentially had to convince Squall (the player character!) to date her. Once he accepted it, the romance really meant something to Squall, and it completely changed his outlook.
In Divinity: Original Sin, you choose to let a companion join your party, or you don't. If you don't invite them to your party, then they just stand around doing nothing. If they join you, then they will talk sometimes and maybe disagree with you (and maybe leave the party if you make them too unhappy), but they never do anything particularly meaningful to the plot.
The problem I have with Bioware romance is that they had to keep a lot of stuff generic. Commander Shepherd never has a chance to be introspective about his romance with Liara. He never thinks about how strange it is to fall in love with an alien. I make the dialogue choice, romance ensues, and then Shepherd mostly forgets about it when he's going about his business with saving the world. Shepherd is surrounded by characters with feelings and motivations, but for the most part Shepherd is a blank slate. I rarely hear him express his feelings. I tell him what to feel.
Companions in D:OS are even less important to the story than in Bioware games. It's hard to design significant plot events based on particular romances in the story when it's possible that character might not even be in your party.
I think the best we can hope for is something similar to Bioware style romance, but without the awkward implication of "sex as reward". However, I think even asking for that is asking for a lot. With the "love and rivalry" thing, it really sounds like there will be a broad range of romantic interests, so I expect we'll be getting something between Bioware and Bethesda (eg Skyrim) style romance. This is what you tend to get when the game is too open-ended.
