Those are all solid ideas. As you said, this style of game could very easily get out of hand in terms of its scope. As such, as I try to plot out the game, I think I'm going to need to establish a priority list.
My thoughts:
1. There will definitely be a gender-swap option at some point going forward. It's a common enough theme in expansion and MC works that it'd be worthwhile to do.
2. Mythological stuff would be kinda interesting. I could see that as some late-game content; having a holstaur, goo girl, or lamia as part of the cast would be fun. I would need to establish a better base for the game going forward, though, since at the moment I'm doing some revisions of my initial plan.
3. Body-part-swapping or body-swapping could be fun, but depending on the format of the game, it'd be rather hard to do in Twine. Harlowe isn't an object-oriented programming language, from what I understand, so it'd require a bunch of additional variable-juggling. If I were to make the switch to a Python-based system or something similar, I could conceivably do it, but that's a distant goal.
4. I'm gonna be honest, I'm generally into people getting bigger rather than smaller. SPH is gonna be a low priority. Body shrinkage (ie, minigirl/miniguy) is still possible, but I'll need to get a better workflow going for that.
Debug menu is an absolute must, I agree, particularly from a developer-side view of matters. I'll look into how I could set that up in Twine.
I was using the "think about your wife/life/relationship" as a sort of narrative stat menu, but A: I'm kinda struggling making that sidebar mobile-responsive, and B: I do think that having a number-based page would improve matters as well. I'll look into those.