Thanks. After some contemplation, I now realize that the previous post heavily emphasized response and reaction to player's actions. If you do that right, you could be on par with Monochrome Fantasy.
But if you want to take it a step further, there's another dimension you could think along: proaction and npc initiative.
What do I mean? Well, depending on their mood, how they feel about you, what their wants and desires are, the characters could take the initiative to do things (or ask you to do things), rather than merely reacting to the player's actions.
Just a few concrete examples of something that Yui for example could do out of her own initiative:
1. If she's happy and energetic and at least somewhat affectionate, she could ask to cook together with you. This could increase the amount stamina restored and/or increase affection? Of course cooking together could be the stage for additional events to unfold!
2. She could inquire about how your day went. If you lost in battle, she could offer consolation, care, or encouragement. If you spent the day with the other girls, the reaction could be anywhere from indifferent, happy (that you have friends), teasing, or even jealous, depending on her mood and/or your relationship with her.
3. If she's feeling down or lonely, she could ask you to spend time together with her. Various options, some of which might depend on status of relationship. Playing together, chatting, giving an embrace, giving a massage, consoling her until she falls asleep, going to bath together, how about going for a walk together...
4. She could ask you to go shopping togehter, or ask for you to bring a specific item (food, gift, treat, clothes, book?).
5. She could give you a gift or a surprise hug...
6. Obviously there's plenty of things she could initiate when her lust is high...
Some of these initiatives and events could be used to create a chain of actions that make the characters and their interaction more believable and lively. For example, instead of flat out asking you to go shopping together right on a random Saturday morning, she could instead present the request earlier during the week and ask you to promise to go shopping next weekend. If you keep the promise and make her happy, she could take the initiative to thank you sometime later. It's always more interesting if these events aren't just random one-offs that are instantly dismissed, but rather lead to new actions and interactions & positive results somewhere down the line.
In general, having characters take the initiative would break up the monotony of the player always being the only one to take an action and observe the reaction. It gives the sense that the world isn't entirely passive.