Calling it right here and right now. They're going to do a combined Deltarune Chapter 2 Switch announcement and Frisk in Smash reveal trailer, just like what they did with Isabelle in Smash and ACNH. You heard it here first, folks.
Also I think there's a good chance they show something new for Legends Arceus. This Direct is focusing on winter games, after all, and Legends Arceus is Nintendo's most prominent winter release. Pokemon has had a good track record of showing up in September directs, and if it's a mainline game, they will usually show off two new Pokemon as well (Blacephalon and Stakataka in the Sep 2017 Direct, Cramorant and Polteageist in the Sep 2019 Direct), so I think they will continue that trend here.
edit: screw it here's my bingo card