I think an arch hag could make a good long-term adversary for a campaign, with the final confrontation not coming until the party is equipped to deal with them (maybe after a few teases where the arch hag toys with them a bit, always avoiding a straight-up fight). Or they could first appear as a potential ally, dropping clues and/or other forms of aid (always with a hidden cost), maybe some magic items embedded with subtle curses, and the like. Properly played, a BBEG like this could end up as hated a nemesis as Strahd, with an equal payoff once they are finally overcome.