This is going to be a fairly lengthy read, but please bear with me for a moment.
Basically, I think it would add to immersion in the game if some of the subjects (specifically those with whom you managed to hit all the right spots) developed romantic feelings for the player character.
It could be as limited as altering their dialogue to some extent and bearing some weighting on their initial friendliness and overall enjoyment during future sessions.
It could be slightly deeper than that, making them appear a bit more often in the subject selection menu regardless of whether you saved them as your favourite character, transferring them from a research subject to a volunteer or a client with a very loose set of requirements (no time limit, predjudiced to pay more than they would otherwise etc.) for later encounters if they started out as one.
It could be as developed as allowing arranging for those with a crush on you to come to the lab outside of its opening hours and have some time for just the two of you, with no NPCs around. Maybe they have their preferred set of racks and will ask you to lock them there. Maybe they have an extra set of fetishes they would not feel comfortable to share with you and entertain unless they developed a certain level of affection for you and there was no one but you two in the facility. Maybe they would like to reverse the roles, strapping you to the bondage equipment and study your body instead during your alone time.
They could send you a gift once your bond is strong enough, be that a set of clothes, an accessory, or a sex toy that is or is not unattainable via any other means. The latter could be fetish-specific, either from their or your (if you've already had some time one-on-one) set of preferences.
If these additional mechanics were to be introduced, I feel it would be necessary to implement a sort of memory system in place, so as to make them unable to recognize you and perceive you as an entirely different person (prompting questions like "Who are you? Isn't it Dr. Username's shift today?") when you've changed your appearance in the shower too much or imported a different cgaracter. Also, ideally, being hideous to them after gaining their trust should drop whatever level of affection they had (unless Stockholm Syndrome happens, or they like being treated that way). There should also be a tool to instruct receptionists to refuse services of any kind to characters you select, a blacklist of sorts, because seeing the subjects you don't enjoy seeing just because they are particularly clingy is annoying.
I understand all of this takes a lot, a huge lot of effort to code and might be irrelevant to a game of this type, but I would be extremely happy if anything from what I have described above, or indeed just something similar, made it into the game in any shape or form. I believe it would add to the existing interactions and make the characters feel more real and "human".
Apologies for any grammar/spelling mistakes I've made, I'm not a native speaker
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