So I was playing with the character editor (thanks for pointing it out!) and it's great, but I can't figure out how to insert them into the game.
Also I am confused: Sandbox mode just skips straight to sex scenes. I figured it would have been more of a "free roam, Sims(tm)" mode where you can skip the story and just do what you want.
The story is fine so far but I can see an issue with it - because it is linear and on rails, it doesn't give replay value. Meaning once someone has finished it there is no reason to ever start over with a new MC. In fact, considering the story's length, it is actually discouraging replays.
So here is my suggestion: Change what is currently called "sandbox mode" into "free sex viewer" and then add an actual sandbox mode where the MC can just do what they want Sims-style. Walk around, go to work, eat breakfast, pick up girls, etc, with the story parts absent or at least put on hold until the player chooses to do the next element.