If they do add Jack, I bet you anything he’s friends with Miguel. Because if Miguel isn’t friends with the MUSICAL SKELETON I don’t know who will be.
The story could be something like Jack finds out about the Day of the Dead and is immediately intrigued, but rather than cause another fiasco like he did with Christmas he decides to ask Miguel about it
Had Sully not just gotten a new friendship with Mickey I would have also guessed at him and Jack having a friendship, where Jack tries to get Sully to help scare people on Halloween or something. I guess anyone “scary” could have that one, but Sully would fit best because his job is (or was) about scaring people too
If he comes out for Christmas I imagine his other friendship will have to do with that holiday in particular