I might not have written this exact scenario without the prompt, but ... I rewatched the series and movie pretty obsessively there for a couple of years, and even wrote a rather long essay about all the Mal/River interactions, and came to the conclusion pretty early on that they're already equals enough she doesn't have to be the one initiating pursuit for a relationship to work out.

I understand why it ends up in fandom that way a lot; the age difference is pretty significant, and she's been wandering around with an altered state of mind for most of canon, which desexualizes her. So waking up to womanhood in a hurry-- her taking agency of that and celebrating it, it makes exploring relationships with older guys less problematic.

The thing about River, though, is she isn't just a normal albiet telepathic teenage girl now that Miranda's out of her head. She's not like an X-Men character (well, unless you're talking someone like Erik Lensherr.) She's very deeply traumatized. And when I started thinking about why Mal might be the one initiating, all these reasons why she might be reluctant to allow any more invasions of her person of any type, desired or not, popped right into focus.

That may be why I haven't written more of this pairing, despite my love for it: my construction of it in my head has gotten really complicated. Though it was fun to touch on in Strangely Comfortable... and if I ever get Book's Legacy that far, I think there'll be hints of it. I'm enjoying building their interactions there.