I hate to break it to you, but Leo, Jin, Joseph, Ridge, Koku and Runan are the only ones we can get close to. I'm pretty sure adding another character route would require a lot more time and work as it would drastically alter the story and any other events that occur.
And let's not forget that they would require their own CGs, and all that other stuff since they have their own unique histories, personalities, and many other factors.
So yeah, as far as the game itself and the development process goes, I can't really see any additional routes being added, even if it were somehow possible. But one can imagine, right?