I like all of the directions you're taking!

Since you're still working on the story progression and the hints on where to find girls, I had a couple of thoughts...

1. I like the idea of sneaking into a house after everyone is asleep. Maybe one of the girls can have an unlock event after midnight. Maybe you can sneak into Rin or Hikari's room and you can fool around with them while they're asleep. You make a lot of noise, so eventually their mom wakes up and checks out what's going on, triggering an event between the mom, daughter, and you. Or vice versa, instead you fool around with the sleeping mom and the daughter walks in. Of course there'd have to be a pretty good hint on how to find this event.

2. I thought about how you could have more minor influences on each of the girls. Like separate from influencing sexual activities, there could be ways to make girls do things that don't effect the plot or obedience at all. Like what clothes they wear, and in addition to that maybe adding a bigger wardrobe for each girl. Some other side influences could be things like making them dance or do jumping jacks or something embarrassing like that. Maybe with each new obedience level, one new side influences could be added.

3. As you have more obedience over the girls, you can suggest things that change their behavior. An example would be suggesting to the principal that the school uniforms should be changed to something more revealing, or similarly convincing your mom/sister that clothes aren't needed around the house. Or telling Rin or someone that they need to practice masturbating, so now every night at let's say 8PM, you can walk in on them doing the deed and it would be similar to walking in on them when they are going to the bathroom.

I'm not sure how deep you plan on getting with this game, and I don't know how much time and effort it takes to implement this type of stuff, but thought I'd throw some ideas out there.