I will pay my money for such a DLC. Or your money if you want to give me some money. Whatever, either is fine by me.
I like the idea of permanence in these romances, and although I understand others would like something else from DLC (and I'm not saying "nope, just marriage for 1.99"), I'd still jump at the opportunity if it arose. For me, part of the anxiety that arises post game is that never-ending wonder of "what are they doing now?" and this is especially pronounced when it comes to the Inquisitor (or whichever protagonist) and their LI. The fundamental drive for me is that I've spent a good number of hours investing not only in these characters but their relationship(s).
So, if I'm going to see them again and have a chance to further develop the relationship, then as long as I'm aware of that (as in, am informed by those actually making the game), I wouldn't really need a marriage DLC - I'd still like it. However, let's say that Inquisition and like DLC is the last time I'll get to see my Inquisitor and their LI, then I'd really like to leave it knowing that "they" (except for you Solas romancers, you're in a real pickle), are happy together.
Whether we see Morrigan again or not, some of the nicest, most satisfying moments for me in Inquisition was her dialogue with my Inquisitor about how she and the HoF are really quite happy with each other and their lives. So long as whatever comes next isn't hugely tragic, that conversation achieved the kind of closure that I've always thought necessary for Bioware's relationships. With my Hawke-Merrill romance, although it wasn't as satisfying, it was still comforting to hear that their relationship too was still a basically happy one.
Thus, I'd be quite pleased and very interested in any DLC that either on its own or as part of a grander narrative (the latter seems more likely and more palatable) allowed the Inquisitor and their LI (still not Solas though) to marry or in some way display that they, like HoF-Morrigan if so chosen, Hawke-LI, and to an extent even Shep-LI, will stick together.
That's just me, your experience will vary.