Yeah, I guess having the same character again would be weird, since that was an entirely separate story.
All that stuff about the Voodoo game seems great! Especially the part about the "you can go full-throttle right away but it makes them leave and not give you more stuff to work with" It adds a lot of realism to the game, and having a steady progression is also good. Although, I do worry a bit that some people might miss that, even if you make it clear in dialogue and such. I think a good way of doing it would be to include a "tutorial character", I was thinking it could be the voodoo shop's owner letting you test out a doll/other voodoo thing on her to help get you the hang of things, and if you go too far with her, she'd scold you for it, so that you can see the consequences in action before going into a proper "assignment". And it'd be fun if the shopkeeper then became a proper voodoo-able character later on (like, maybe her lower body would be hidden under the counter for the tutorial, but you'd get to see it all later on), maybe at the end of the game. I always love it when games do that.
Totally agree with you there, save the backshot-only for later on in the game, it wouldn't make the best first character, but I'd love to see that eventually. And same for TFs, they seem like something to introduce later on. Being able to x-ray through layers of clothes also seems like a good idea for the "thing" to introduce with the first character, regardless of whether or not you're planning on carrying it through for all the other characters. I'd personally prefer if it was one article/layer of clothing at a time, rather than one item to full nude. Something like an "enchanted lens" would be a good generic name, but if they were tied to the personal items, it could make sense if it was a mini dress-up doll/mannequin that you'd get miniature versions of their clothes on, thus allowing you to make the girls' clothes invisible to you by taking them off.
Being able to focus on specific body parts would be cool, especially if what kind of toy you were using connected to the angle you were looking at the character from. But even if not, it'd still make you feel like you had a lot more direct control, especially if different points of contact had different reactions from the character (like, sweet spots). Were you thinking it'd be one single toy per character, thus with each character getting their own body part focus? Or each character could have multiple different toys? I feel like the former would feel a bit restrictive, like you're missing out on something if you really like a whole character, but you can only play with their boobs because that's just the toy they're assigned. Maybe all characters would have a base doll, but then varying "focus" items that you could use to "zoom in"?
Not entirely sure what you mean by "empowering" the voodoo doll, like, making it give the patron more pleasure, or unlocking new capabilities somehow (like, penetration, or the character's unique thing (ie. lactation or TF)) But the escalation of the patrons' demands is, as I said, a good system. Maybe the empowering could also let you see more of the character. Like, by strengthening the connection, you'd get a better view of them, magically-speaking. Or, in terms of gameplay, that means if the character was, like, sitting behind a desk that was hiding their bottom half, improving the connection would let you see through the desk.
And, "Voodoo Shop Assistant" seems like a perfectly good name. I can't quite think of anything better. Most people probably don't know what poppets are, so putting that in the title isn't the best idea. But, you could include that somewhere in-game, like that's what the dolls are referred to in shops and stuff. And I agree that most people think of them as voodoo dolls. You could include some dialogue from the witch in charge of the shop making a joke about "oh, technically it's not actual voodoo, but that's what gets customers in the door", so people know you know they aren't actually voodoo dolls. And, "Witch's Assistant" seems a bit too generic of a name. You could do something like "Dollmaker's Apprentice", maybe? But that gives off a bit too much of a mystic, mysterious vibe than I'm getting from the rest of the game concept.
And, about Hypnothick: Oh, an alt ending! That makes sense as a way to expand it. Any sort of new TF attached, or just the ending?