I wonder if they would ever do a companion that was down for casual sex, but would never progress into a romantic relationship as we know it. That's kind of what I would expect from Iron Bull, if they'd do something like that.
I think Jack is the closest to that. You can sleep with her immediately for carnal desire's sake, and the romance cannot be progressed beyond that.
I think it could be interesting to have have a fling or two available with non-LI companions, if written with a purpose and offered something to roleplay and the character companion.
But really, I'd like it if all of our companions had flirt options available for roleplay's sake (a la Aveline/Morinth/Samara/etc). Even if Warden might not be an LI, maybe my PC wants to have the option to flirt. From there, they could get turned down and Warden could have the opportunity to talk about the Calling, and thus why he's not interested in getting into a relationship. Or flirt with Vivienne and she could say, "Woah, there, I got a man back home. We met during my time as... *cue story*" Basically, flirt options can double-duty by allowing us the chance to roleplay if we choose, as well as be a jumping off point for characters to get into depth.