I think that you can be anything you want no matter what your size. It's not your fault the costume industry thinks Halloween is for showing as much skin as possible! In real life, people come in all shapes an sizes so the doctors, nurses, firemen, policewomen etc. are all sizes.
I don't see why you can't be a sexy devil! Who says a devil has to wear a skimpy low-cut leotard? Get red pants or a skirt a red tee shirt, devil horns, a tail and a pitchfork! Have your husband be the angel. It would be a cute turn of events!
Or go as 40s gangsters! Black pantsuits, hats, cigars, big guns!
Zombies? Bride of Frankenstein? Geisha girl and samurai? How about Togas?
Just don't ever think you can't dress as something just because of your size. I think you'd look a lot better than some tart in a skimpy costume!