Why in the world would you want the developers to put time and effort into a romance you've done in a previous game ?
If you want to romance Leliana again, nothings stopping you from playing DAO.
I would like them to develop this character to the point where I (and many others) can romance her. As with all romance relationships in Bioware games, they all further develop the backstory, maybe why they are the way they are etc. We have a character that has already been established within this universe and according to lore, it's now been 10 years since we have had a proper good look at her and judging by the, albeit small, amount of content we have seen of her, she is most definitely not the same as she was in DA:O. In my own small way, I prefer this new Leliana. One thing I found to much with her in DA:O was that she could come across as too nice. I like the fact she has a much sharper edge to her personality now.
Now I admit, this is my opinion, this is my taste and Bioware are not going to drop everything in order to just please me; I get that. But the truth is, there is a large chunk of the community which do want her as a romance and like Bioware have stated before, countless times, that player feedback is there priority, it's how they know they are making the game they want to make, and the game the player wants to play. Now like I said in my opening statement, they don't even have to make it a full-blown romance; if it was like Samara's romance in Mass Effect 2, I would be fine with that. In there, she actually gave a reason why she couldn't be with Shepard and it was fine. And the thing is (I realise you don't start a sentence with 'And') they could add it alongside other content they have planned for release; like I said, they already have everything in order to make it work.
I think what's bugging me is that, we (as far as we know) don't even need Leliana. Bioware could have created a completely new character in order to fill the role Leliana does within the Inquisition. They have chosen to bring back a character and put said character in a prominent role within the game and unlike the other two advisors, they've chosen to close this road off. Why? Because it could cause problems with some players? Well that's adhering to fan feedback right there but then, that's easily fixable within DA:Keep. (I know that "Because it could cause problems with some players?" is not the only reason but it is definitely up there). Using the Keep in the way it was made for would fix it because you could say, 'my Warden didn't romance Leliana', well right there you've fixed the whole issue with unfaithfulness or whatever your reason is. There is loads of content that has been built and waiting for us to explore in what, less than 40 days nows, but a large portion off that won't even be touchable due to how your world-state is.
You are correct though, I could just go back and play DA:O again, I still own it but what about those who don't?