Ahasuerus is generally identified with either Xerxes I (519-465 BCE) or his son, Artaxerxes I (465-424 BCE). Xerxes I is best known in the West for launching the second Persian invasion of Greece, which resulted in the legendary battles of Thermopylae and Salamis. That means that such figures as Leonidas, Themistocles, and Artemesia could have been contemporaries of Mordechai and Esther. It’s even possible (though not very likely) that Themistocles or Artemesia were present for the events of the Purim story, if they happened to be visiting Shushan (known in the west as Susa) when shit went down. Therefore, I see no reason why they would not be appropriate subjects for Purim costumes.