What's strange is that "regular" incest stories do quite well here. I think the top-viewed story on the site is about mother-son relations. It's one of those mom sits on the son's lap in the back seat while the family in front is oblivious tropes. That's almost always a surefire hit. But it's hard to take any incest story as anything but humorous. (Well, there is Savage Grace with Julianne Moore, which is a tragedy.)
I wonder how a father-son body swap would fare.
There's a 1987 movie about a father/son body swap called 'Like Father Like Son' where a father played by Dudley Moore and his son played by Kirk Cameron swap bodies but I'm not sure if anyone has ever written a Literotica story where a father and son swap bodies or a mother and daughter swap bodies like in Freaky Friday.