According to Halloweencostume.com:
(1)Plush Big Bird Costume
Trend: Political commentary costumes are huge due to it being an important election year. Masks for both presidential candidates have also been super popular.
(2)Minecraft Creeper Head
Trend: Video Game Costumes. Whether nostalgic video game costumes like Mario characters and Link, or contemporary video game characters like those from Minecraft, this trend is hot! We've had trouble keeping these "creeper" heads in stock since Minecraft is a super trendy game both for boys and adult men.
(3)Deluxe Dark Knight Catwoman Costume
Trend: Superhero costumes, especially those corresponding with movies superhero films that premiered this summer, like Batman and The Avengers. "Catsuits" are also a very popular costume silhouette this year, so that makes this costume a double win!
(4)Child Monster Costume
Trend: Monster Costumes! These are fun and furry for the whole family! There are styles for babies, children, mom and dad, and sexy ones for adult women. The fabrics are soft, and colors are bright, and everyone loves the hoods.
(5)School Girl Zombie Costume
Trend: Zombies. "Gory Glam" is in this year. And with zombie attention from the media through both popular shows like The Walking Dead, and the zombie apocalypse, it's the year of the zombie.
(6) Ninja Turtles Costume T-Shirt
Trend: Costume T-Shirts. Again, these are awesome since you can accessorize them like crazy for Halloween, and then wear just the T-Shirt to school or work.
(7)Yoda Dog Costume
More people are dressing up their dogs in costume than ever before, with Americans planning to spend $370 million this year on pet costumes alone.
For more costume ideas: