24 hour clocks in games rarely work well, its too difficult to find the scenes. I don't think you need to change the mechanics again for just 2 characters, sure you won't get much done in a day but I'd rather play more days than have to run around in a 24 hour game trying to find stuff. More interactions could be nice, but if there isn't a reason to do them ( progression of the story ) there isn't a reason to have them IMO. Its just filler, when you do it once why would you do it again ( unless its a repeatable sex scene) ? People tend to really dislike these sorts of sandbox games, its too tedious.
Right now there are 4 times in the game. Maybe expand that to 7 or so, thats about as much as I think could work and not become too tedious.
Early Morning -> Morning (Tiff school and Lucy theater) -> Noon (Tiff school and Lucy theater) -> Afternoon (Lucy theater/Tiff school) -> Evening -> Night -> Late Night
That gets everything you want, but its still compacted and simple.
Addressing a few of your reasons for doing this 24hr system
- As I posted above, I think players would rather play more days than have to slowly advance through one at a time. Its too tedious.
- The time system I posted does what you want for Tiffany/Lucy and its a lot more compact.
- You say "you'll get tired of always going to the same place where nothing is changing. " but won't this system just compound that? Doing a different event for 24 hours is way too much work. You can change their interactions based on their progression and implement a few different interactions that happen at the same time (would be randomized). See below.
- "That's why I also want to make more interactions because right now, nothing is happening in the game. The same empty locations. "Everyday in the morning Melanie is sitting in her room and doing nothing." I also want to make the game world feel more alive. I want to fill it with different NPC. " You can do all of that without a 24 hour system. Going to list some idea things below.
- Going back to the previous, implement random interactions. Maybe some mornings Melanie is in the room, maybe sometimes she is fixing breakfast, maybe sometimes she is watching TV or cleaning etc. This would change with progression, maybe she even dresses differently as her story goes on. You can do this for each of the 7 time frames, would add a lot of change to the game.
- More NPC, sure maybe put some customers in the coffee shop. People hanging around the school or whatever. This would also be randomized, maybe one day there are some cops eating donuts at the shop and the next a family is there for breakfast. You could do some small interactions that build up over time too with them.
- Implement a "dating" system, this unlocks once you have progressed with a girl enough. This adds actions and extra events/interactions you can do. Take her to a restaurant, to the mall, bowling etc.
This stuff I've went over does everything you want, but I think its a lot simpler and better IMO.