Well, going as an Alpaca would certainly keep you warm….otherwise, how about (and I'm probably going to get a bit offbeat/obscure here…)
Arthur? Murray used to go as Arthur to every costume party. White shirt, yellow jumper, and jeans, and maybe some
bear aardvark ears. Done!
chuck on a green jumpsuit (or just a green jumper and green jeans/leggings) and go as an alien?
a plaid shirt, jeans and an (optional) work belt and fake beard and go as Al from Tool Time/Home Improvement.
Agnes Skinner would probably get a few chuckles and be very warm
Anthony Hopkins in Silence of the Lambs?
Annie from the musical
Anonymous (as in the actual collective)
Amy Winehouse
Amelie
Queen Amidala
Alice in Wonderland? (boring as fuck and not super warm but probably easy)
Ann from Parks and Rec (also boring but probably easy)
The Anaconda music video (okay, that's more of a summer idea….
)