Sigh, all right, it seems some of you folks got me quite wrong in one or the other point:
First, I know of DA:O and the option to romance Leliana in it, and some of my warden did. And I really adore Bioware for creating such an interesting character. What I thought of in my little construct was to have a different point of view on her from a different angle, by a different character in a different time under different circumstances. I don't think even if there was a romance between her and the inquisitor, that it would be in any way comparable to the on between the warden and her. So, to remind me of DA:O and its romance options for her is to a certain point fruitless.
Second, of course I did not mean to create a DLC only for the romance. Bioware proofed with its Shadowbroker DLC for Mass Effect 2 that they can implement a romantic storyline into an action packed DLC, so there is no need to worry, they would waste resources.
I think you don't really understand the reason why previous love interests will never be love interests again. It's not because your former character may or may not have romanced them, and it's been ten years, and your Warden this or your Warden that - it's because everything in a game has a cost. Character arcs and romance arcs cost word count, animation, cinematics, all kinds of assets; and these assets are not infinite. If you get resources spent on a second Leliana romance, you get it at the cost of resources spent on something else in the game, not in addition to it. Let's say, in example, the Cassandra romance. One in, one out, now you'll never experience this whole new romance, because you want to relive through an old one. You don't get to have the cake and eat it too.
They've actually been pretty straight forward about this: development will not waste their zots telling the same story over and over that they've already told once. Not when they can move forward and dedicate those zots to new characters and new stories that haven't been told yet. You will not get a second version of how a Leliana/Morrigan/etc romance would have been in a different time with a different hero because their stories have already been told, and the game gains nothing (but loses a lot) from retelling them time and time over again.
I can follow your line of argumentation though it makes me sad to be reminded that creativity is so strongly limited by simple resources. In my view Leliana is too interesting in her personality to be laid "ad acta" after DA:I but I can see, that most of you are willing to draw a curtain to her storyline.
After all this was just a spontaneous idea. 