I think this would be more compelling if, for once, it was the other way around. "The demon lord/demon lord's daughter secretly loves the hero" has been so overdone I immediately predicted it from the start. It's played out.
Let's see the inverse. Make the hero the one who's secretly enamored with the demon lord's daughter, but ashamed of his feelings for her because, well, she's a demon and thus his enemy. But then the demon lord comes to them with a proposition and he accepts as it's his duty, while also secretly happy, but still very reluctant because of his conflicting feelings and his knowledge that this is just a political marriage from her part. As for her, SHE'S the one who is just "doing her duty" for the sake of peace, and slowly falls in love with him and humans as a result.
I think that would make a more engaging plot than making the girl being the one holding the torch for the billionth time. And, yes, it's possible that the hero will also secretly having feelings- but even if that turns out to be the case, it's still not as engaging imo as a one-sided romance where the heroine has to be the one won over, for once.