I doubt it'll be Maleficent - she'll probably be the last character we ever have to welcome at the end of the game. It could be Lady Tremaine, since she's one of Disney's most iconic villain's yet still isn't part of the game, or maybe Madam Mim to go with Merlin. I think the only villain missing from any Event set is the Evil Queen, so possibly her? Maybe they'll be drawing inspiration from the park's Not So Scary Halloween Party and have these villains come together for the witches of Hocus Pocus? They are pushing that movie big time right now.
Edit: totally forgot we already have the Evil Queen, so I really don’t know what limited time event villain we could get.