My thoughts on the concept are that I'd want to stick fairly close to the source, with the deviations being hypno focused. If I were doing it, the Pacifist/Genocide split would remain, but renamed as Charm/Enslave. Instead of kill or be killed, it's a dom or be dommed type thing. Monster attacks bring you closer to being enslaved, while Frisk's "fight" option enslaves them. Since Enslave is the stand-in for dying, it involves personality death as monsters become Frisk's puppets or he/she becomes theirs. Or, you can do things peacefully with the "mercy" option, which will Charm them, leaving personality intact and friendly to you.
Determination stays mostly the same, but allows Frisk to reset after being being enslaved. Anyway, some major plot points in a Hypnofied Undertale:
-Toriel still wants Frisk to stay with her, but tries to enslave them to keep them safe when they try to leave. She still can't bring herself to actually do it and is horrified if she does accidentally. Toriel lets Frisk leave after being charmed or enslaved.
-Papyrus still wants to capture the human and employs a large number of HYPNO PUZZLES for the task. Like in the base game, Charm Frisk completes the puzzles and Enslave Frisk ignores them. Eventually during the fight Papyrus is either enslaved or the annoying dog steals his Ultimate Spiral and he gets charmed.
-Undyne tracks down Frisk and attacks with her magical spears. I'm thinking the spears could be hypnotic drug needles or brain injection equipment. In the Charm storyline, the same chase through Hotland ensues, culminating in Frisk splashing Undyne with magical and/or drugged water to charm her.
In the Enslave story, Undyne takes the enslaving attack meant for Monster Kid and, rather than succumb, transforms into Undyne The Unbroken.
-Muffet attacks and tries to enslave Frisk so she can use their money to benefit spiderkind. She ends up being charmed when the spiders at the ruins bring her the message that the human didn't enslave any spiders. Or she's immediately enslaved in one hit. Either/or.
-Mettaton's game show proceeds with tons of spiral screens and hypno puzzles. I don't have a concrete plan for every minigame, but like the thought of his Enslave route transformation being called Mettaton TRANCE or something.
-Asgore. Start of the fight he destroys the "charm" button and goes full slave master. He didn't want to do it, but he's already enslaved 6 human souls and can't stop now. Hope you understand; perhaps your mindless body will enjoy tea.
Naturally this will move into Flowey's coup and nightmare form.
I'll think more on the True Charm an Enslave endings later, but I hope these ideas are enough to get some conversation started.
