I think you can get somewhat of an idea of what costumes will be hot this Halloween but a look at what's happening at Comic-Con(s).

If my assumptions are correct, expect Rick & Morty stuff to lead the charge this Halloween season. Superheros continue to be big, but not the ones you expect. It seems to me that I saw more Spiderman and Superman than any other costumes (you'd expect more Avengers & Black Panther).

You can also expect a BIG surge in costumes based on Fortnite, Overwatch, and anime.  It seems like the folks who make costumes are doing a much better job of keeping up with current trends.

As for the ladies, I don't think Harley Quinn is done yet, but there seems to be a surge in the video game iteration of Cat Woman (ladies love cats+the black is flattering).

Lastly, I think Star Wars is always going to be a thing.

Overall what I'd like to say is, that costumes are becoming more intricate and more personalized. People are willing to wear more oversized and outrageous stuff and that's just fu for everybody.