As an alternative to vaginal, I suppose it's technically possible, but you'd need to define the slit's looseness, wetness and capacity (since many scenes make reference to at least one of them), make a new function for when a load gets blown inside it, and modify every single scene it's to be implemented for to accommodate it. There's also probably more I can't think of right now, so it's probably too much effort to be worth it, and I doubt we'll ever see genital slits as an alternative to vaginal.
As unique scenes, however, it's easy; someone just has to write them to the standards required to get in the game. The only difficulty I could think of after that would be figuring out what to do about an equivalent to the loadInCunt() and loadInAss() functions.