Idea dump:
The ability to give Cait multiple breasts, or hyper breasts, through either a cow TF or some other item.
The ability to further corrupt Eryka into a succucow like Kasyrra or into some other kind of demon. Potentially gaining her as a follower? Could also potentially alter the dream sequence you can sometimes get a bit. Potentially could transfer the transformation onto yourself permanently if you want to be a Kasyrra clone/look-alike.
A Bimbo Liqueur/Bimbo Liquor like item that you can trick NPC's into drinking to change them. (Berwyn, Cait, Eryka, and Gwyn come to mind, though it could be given to just about anyone. If there's an "insanity" mode or "silly" mode, give it to Kasyrra?)
The ability to transform into a succucow like Kasyrra by having sex with her enough or some other means.
A corrupt counterpart to the goddess fountain. Or the ability to corrupt/purify it with the right items.
Combo actions with a specific party setup. Like three magically oriented characters being able to cast a "super spell" of some kind.
A "sneaking" mode to reduce encounters while traveling? Could also add a "rampage" mode or something to have more frequent encounters.