Would love to see more emphasis on damage. If something tries to enter the witch but she's already "too full" of mana, would love to see some red irritated skin around the cleavage and a look of uncomfort from the witch (or maybe something positive at larger sizes as she seems to enjoy the mana)
Would love to see events in which mana is forced on the witch. Would love to be able to "train" the other witch. Would love to see a cutscene of the witch "training" to take on more mana. Little things like this as either cut scenes or just more art really. The game is solid enough already (though as others have said, a tad grindy) and there seems to be a clear identity as to what it is
It would be great to unlock a 3rd mana storage (butt) but that's my own bias speaking.