Boy or girl? Shirley Temple for girl (little girl dress, dancing/party shoes) Rev, Awdry who wrote the Thomas books. He can wear either an engine driver's uniform, or Thomas T-shirt if he has one and carry a book.
Well DD would probably choose to go as Boudicca (tartan blanket as dress, face/arms painted blue - although that's probably incorrect), or Joan of Arc (sword, shield, trousers), or Cleopatra (white dress, toy snake).
Boys could be a viking (hat with horns), julius caesar with sheet as toga as PP suggested, if they have a sword and shield set they could be Sir Lancelot or King Arthur, a victoria chimney sweep with black top and trousers and a sooty face (social history!), Rober Peel or similar with a black top hat made of card . . .
DS would have gone dressed as a knight ( St George, if you ask him which knight). I think there would be fair few Vikings, warriors, princesses, kings and queens too.
Cave man/girl could be fun. Davey crockett if you have raccoon type hat and brown trews and shirt. Neil Armstong - if you have a a space man dressup suit. Pick a famous heart surgeon if you have a Drs dress up kit. Famous ballerina- leotard and ballet skirt.