The reason for the sandbox modes is to let the player find out the secrets of the people he meets and decide which ones he wants to exploit. So it wouldn't be a bunch of repetitive grinding. For instance, MC sees his wife doing yoga one day and realizes he's getting a bit out of shape himself as well. Later that night he says something about it to the wife who says she's been thinking about getting a personal trainer to help get back into shape. He can say they can't afford it or something in which case she'll continue doing her yoga solo. He could let her get a trainer, in which case she'll hire a male trainer. He could offer to get her one himself, in which case he could get a male or a female trainer. He could continue to let her workout alone or suggest they work out together. He could tell her she already looks fantastic and he should be the one working out in which case she stops working out and ends up being the one who catches him with the trainer.
Then a few times a week or whatever you get to see the workout and depending on what MC chose to do, either wifey works out solo, she works out with a male trainer who gets increasingly inappropriate, or she works out with a female trainer who does the same. Or if he tells her she already looks great she could be the one who catches him working out with the trainer. A male trainer would only have the potential to get anywhere with her but a female trainer would potentially go for either or both of them. The scene would escalate further each time so at first it's just the trainer getting a bit handsy with some entirely unnecessary "form adjustments", and eventually leads to them fucking someone.
The idea is basically to start with a relatively tame scene where MC discovers something about someone, he chooses what if anything he wants to do about that info and potentially uses that to escalate that situation towards whatever he wants. So using the above scenario as an example still some players might want MC to cheat on the wife with the female trainer, others might want her to cheat on him with either the guy or the girl, and still others might want the wife to get double teamed by MC and the trainer (whether it was a guy or a girl).
I also wanted to have it set up so that your choices do lead to significantly different scenes but most of them culminate in something hot so there's a lot of replayabillity in trying out all of the different options (or skipping ones that you just have no interest in seeing). I basically wanted at least 2 or three significantly different potential outcomes for each character for each starting event. So a lot of the sandbox events would be fairly complicated and repeatable.
