but being able to have a date with Hawke where you control both characters in conversation
Sounds a bit like masturbation, tbh? Or some sad forever alone scenario where you go on dates with... yourself.
But speaking more seriously, no, I don't see much appeal in this. A lot of the appeal in the romances we have is that we don't know how the NPC will react to the things we do or say, that they have lives and personalities and beliefs of their own and sometimes they won't like something you do, and sometimes that'll be the reason they'll fall in love. That's how you date in real life, right? You don't control your significant other's thoughts, you don't choose what they'll say and when. That's half the spice of a relationship, the trial and error. Controlling both characters in a romance would feel like puppeteering the relationship, and I think it just wouldn't be as interesting - if I can control both characters, I know exactly where I'm going and there's no spark to it.
As for romancing Hawke in general... maybe if they didn't romance anyone in DA2, or if their LI is out of the picture due to shanking or coat-turning, I can see it. Although that would've taken a huge investment in time and resources making Hawke a more significant presence in the Inquisitor's life, and that would no doubt have cost us in other places, probably in having less companions and romances elsewhere. I'm not sure anyone who was really into a romance in this game would be willing to trade its existence for a Hawke romance.
Nothing says you can't headcanon your Inquisitor with your Hawke, though - people ship their Inquisitors with Minaeve, Barris, gender-gated companions etc all the time. Canon is just a springboard for roleplaying, in the end.