I recall the devs sayin that the Underdark is/can be/should a part of Act 2. So if the Hag will return, maybe will do so thru her portal to the Underdark
If you are a wizard you can point out to her that she is trying to cast the Hag's Eye spell on your eye (you can even find a Hag's Eye amulet somewhere in her hut), to which she replies that you get to keep your eye in your socket as she has no intention of turning it into an amulet. I think that she turns it into a Sending Stone like Mizora did to Wyll's eye, but that does not account for the possibility of you Sparta kicking her into that pit where Mayrina is. If you kill her in her hut though her corpse hints that she can just revive herself in a later Act.
Correct me if I'm wrong but there are some letters in the Ethel's inner sanctum that are correspondences with a sister, so instead of her reappearing it could be that we'll be meeting another Hag with how we dealt with Ethel affecting how she views us.
