Those are some good ideas. I probably won't deal with any other parts, even though butt is certainly viable, because I believe in the concentration of kinks. A game with full focus on one thing as both the means and the end is highly focused and tends to be rather hot, whereas one that lets you go in fifteen different directions tends to lack depth and dilutes the horniness for me overall.
I'm thinking of using the "abilities" system some idle games like clicker heroes do, where it's additional things you can do with a cooldown. The one I've already implemented clothes the fox in question, decreasing growth while the bra is intact, but as you grope her she visibly squishes more and more against it until it snaps, making the fox grow twice as much as she would've otherwise. Like the Claustrobobia images on my FurAffinity gallery.
Gonna make a human head a setting, since that's about all it'll take to accomodate. Easy enough. And if they can't even stand animal ears and tail... well, too bad, I'm still the artist, author, designer, and programmer extraordinaire. I get to choose what to make.
I was thinking of making a character gallery to view foxes you've already grown.. A set of japanese syllables, two or three of which are selected at random to form a name like "kukiko" and hope that nothing that's generated is too offensive in the language. Shrinking is against my and the foxes' belief system, hence the new characters when you need a smaller one. Descriptions, moods, and personalities will take a lot more writing effort, but I'm sure I could slap something like that together.
I'll probably also add a creative mode, maybe unlocked by password or at the ending, where you can just customize the exact things you want on the fox rather than having to deal with a power rating that's too high and such. The power rating's something entirely temporary, by the way, I'm going to use a system of upgrades like I did in Yuri My Beloved.
Nevertheless, this is the most detailed response yet. If you send me your discord username, I can show you the bra prototype I've made. Not enough change for a new release, but enough to get feedback on from a few beta testers.