Let’s narrow the range a bit.
• Do you want a female character?
• Do you want instant recognition? A man in a suit really isn’t going to be readily identifiable. A lot of contemporary figures don’t have any special way of dressing that tells you who they are without a face or an acted role.
• Do you want to have to wear a face mask? Let’s just say no to that right up front. That’d be pretty awkward for a day at school. So would carrying a lot of props.
• Do you want something you can accomplish without a lot of very specific costumery? For example, Marie Antoinette was just a woman underneath all the silk and lace gowns, the cosmetics and wigs, and the spectacular jewels. If you just put on a nightgown, you could be Marie Antoinette at bedtime, but no one would know that. It would take a lot to dress as an 18th-century French queen.
• Can you afford to spend money on this?
• Do you want to be a real person? For a real person, you may have a familiar (even iconic) portrait to refer to that people will recognize, but then your audience will have to know the reference. Otherwise the history must be well known; for example, you could wear a cap, a long dress, and an apron, and carry some red and white striped material and a sewing needle, and people who’ve studied America history would know you were Betsy Ross. A fictitious character either relies on evoking a work of fiction, which can be hard unless you can incorporate something symbolic, or else requires you to impersonate not a character but an actor who’s playing a character.
• Do you want to be a person of the past or present?
• Do you want to do any kind of impersonation or just go on sight? For example, if you were the Queen of Hearts from Alice in Wonderland, you might have to say “Off with his head!” a lot to dramatize your character, whereas Alice or the White Rabbit can be identified on appearance alone.
Once you’ve set a few limits, it should become easier to zero in on a character and costume that are actually doable for you.