There are so many games that are a lot of fun to play and they have good writing, but lack animated sex scenes. How difficult is it for a dev to go back and insert extra animations into an already completed game? I've barely played around with renpy on Linux, so I don't know how it works yet. I have no idea how the other engines work.
I'm not a coder, so my opinion isn't worth a lot, but I wouldn't think it would be terribly difficult to change the button that starts the sex slideshow image request into one that calls up a video file instead. But as I said, not a coder, so I could be way wrong.