I’m going to stop you right there though - we have no true confirmation of the villains coming this October or even being playable characters whenever they do come. And similarly Jack isn’t completely deconfirmed from coming this October either - I don’t seem to recall anyone assuming Hiro and Baymax were coming in September, the only one from their update dataminers seemed to know about beforehand was Moana and she had been hovering in the “coming soon” speculation for months at that stage.
So basically, don’t rule anything out, they could still surprise us on this. And if Jack isn’t here for Halloween, hey, there’s always Christmas.
In fact, the original poster said this year was the 25th anniversary of Nightmare before Christmas, right? Wouldn’t the film have come out around Christmas time? I think it might actually be more likely they’ll do a big update for Nightmare around December