Tonight's magic word is "Consistency". I'm gonna be using this word a lot.
That's fair, I would like to point out that those images are concept images.
Marketing Advice 101: Don't Put Anything in Your Customer-Facing Page That Can't Be Considered "Final"
AKA Don't stick the concept art in the shop/download page unless you're trying to get funding on kickstarter.
That's not to say you can't change things later as you iterate, that happens a lot especially with longterm projects. But your audience right now wants to play a porn game, not concept art.
As far as the scales and horn appearance I will address that in the game. Her full succubus form will be more dragon esque, her horns more elongated. When she's actively using magic or sucking out souls she will lean more into that form. So, a change in scales and horn size will be intentional while also giving me more leeway with my generations.
Gonna advise against doing too much variation because you could just end up creating more of a headache for yourself later when you try to get newer images to match older images. From my experience AI generation as it stands now has a terrible time in handling consistency for characters with drastic differences between "forms." You really need a human artist to be able to handle that.
What I would advise is nail down here human/close to human form NOW and make sure it's consistent with what you put on your title page. Consistency is very, very important for selling an idea, whether it be for money or popularity.
Moreover don't let perfection get in the way of being finished. If you can only manage one consistent design for Allison despite wanting her to have two or more, just do it that way FOR NOW. You can iterate later.
I'm glad you aren't put off by Ava though because I tried to make sure she was consistent since this is her chapter. Also, her hair crossing in the middle is a defining feature she has which is present in all her generations even these conceptual ones.
Not to keep banging the same drum since you're probably sick of it by now, but the reason I'm personally not put off by Ava is that her design is consistent. Having just ONE unique element that stays the same from picture to picture makes it much easier to identify who she is and what her deal is.
The thumbnail/main page image is quite eyecatching, so I was actually quite disappointed when the succubus didn't have a consistent appearance from picture to picture. If you can manage that you'd do VERY well...assuming you release at a consistent rate.