A couple of general things I enjoy in games:
Progression and build up: This seems to be a fundamental for most erotica, but even the driest gameplay loop has kept me transfixed in anticipation of the next text description or character sprite. As long as the loop doesn’t feel so contrived as to intrude on the experience, I’m happy to be led by the nose and play along, so to speak. With a progression course set, it also gives you a means to subvert expectation for extra spice - a character doing something unpredictable or the session going in an exciting new direction.
Organic characters: good and engaging characters, again, should go without saying in games like these. Outside of outstanding character writing, some of the more memorable games for me in this regard employed a simple trick to convey an organic, natural-sounding character - variable speech triggers. Having a character say different things not only over the course of the gameplay loop but from session to session lends an unpredictability to a character and helps make each session unique and personal to the player, feeling less scripted.
It not only builds a narrative in the player’s mind as to why a character could be chatty in one session and quieter in the next, but also gives a player a reason to replay if they want to see all the character lines, interactions etc. Don’t be afraid of holding stuff back - a dedicated, switched on player will make it their mission to find it!
Both of these play on a more fundamental idea of setting expectations and subverting them, going off-script. That variability is the all-important spice to tease and stimulate the player as they explore and enjoy the experience. Don’t be afraid to break expectations! Most savvy players will be looking to expect the unexpected so play into that.