Going to take a stab at this. This what randomly take to mind, hopefully most are clear. I felt i like sucked at the naming part (any better names?). Some may not qualify as tropes, but i listed them anyways.
Supernatural Assistance - Usually the protagonist is introduced as the average plain guy just living his life (most likely a student) or has nothing going for him. Then, suddenly out of nowhere due to a freak accident, coincidence, or mysteriously he comes across a magical/supernatural being (Demon, Angel, Fairy, Wizard, etc.) or magical item (Ancient Book, etc.), who helps him satisfy his sexual desires with the women in the story via mind control, hypnosis, magical aphrodisiac, superpowers, etc. It may or may not require something in return from the protagonist for using their powers (example: more it's used or more corrupted women, the more powerful they get). Examples: “Dreams of Desire”, “Long Live the Princess”, “The Awakening”, “Sorcery of Lust”, “Dark Neighborhood”, “Sins of the Father”, “Soul of Paintings”, “Harem Villa”, “Seducing the Devil’”, “Betrayal” (mixed with Science Assistance).
Science Assistance – same as above, but all made by Science via drugs or hi-tech items. Examples: “Take Over”, “Shut Up and Dance”, “Betrayal”, "Retrieving the Past".
Suddenly Desired - Protagonist is the plain unnoticeable guy with nothing eventful happening in his life suddenly surrounded by women all interested in him by pure luck/coincidence, he's dragged into a new lifestyle by mistake or supernatural means (supernatural assistance). Examples: some from the other two tropes above.
Black Helpful Friend/Bestfriend - they help the protagonist throughout the story, generally resourceful to the protagonist's goals. Mostly being a computer wiz or is a people’s person. Examples: “Dreams of Desire” (Marcus), “Action Lessons” (Liam), “Sisterly Lust” (James).
Black Douchebag/Side-Villain - usually a sexual predator in the story and portrays stereotypes such as being a thug & speaking slang. Generally having the intentions of making a heroine his sex slave to use however he wants, having the heroine gangbanged, double-penetrated, or pimped out to random guys and if a heroine is involved with the protagonist, attempts to cuckold protagonist in the process. Example: “The Tyrant”, “Babysitter”, “Love and Submission” (think just teased nothing more), “Cultural Shock”.
All young guys are jerks - heroine can’t seem to find a decent guy close to her age, the guys are either horndogs, insensitive douches, immature, and so on. Very common in almost every Age-Gap/May-December romance visual novels of a young heroine & older male protagonist. Usually has a line with the heroine asking why can’t guys her age be more like the protagonist or something around those lines. Examples: “Triangle”, “My Sweet Neighbor”, “Dating my Daughter”, “Parental Love”.
Useless/Broken door - usually the bathroom door in the protagonist’s house for some reason isn’t working properly (broken lock or doorknob, door-less bathroom). Leading to possible run-ins while nude and/or peeking. Example: “Dating my Daughter” (door-less), “My Sweet Neighbor” (doorknob/lock), “Life with Mary” (doorknob/lock).
To peek or not to peek - commonly overused feature in visual novels presenting the player with the option to be peek on heroines. It may or may not have consequences, mostly not. Example: half the games on the forum.
Central heroine/Titular heroine - she’s the focal character of the whole story for the player. She might be only heroine in the story that the protagonist can romance or maybe feature other girls, but not as much screen-time as the central heroine. Other girls are probably to fill-in (The Relief Girl) for slow-burn romances. The protagonist may or may not be penalized for going after other girls, examples like missing a solo scene with central heroine, a game over, or not getting the perfect ending with central girl. Examples: “Dating my Daughter”, “Daughter for Dessert”, “Dreaming of Dana”, “Melody”, “Life with Mary”, “Lancaster Boarding House”, “Alexandra”, “Lexi”, “Our Fate”, “Action Lessons”.
