Well, the obvious would be a guy with 15 girls. But no, too obvious.
Who is that guy at the end of the BOM? Shiz? Go as a headless man, gasping for breath.
Or Ammon. Walk around with an armful of arms.
Or Nephi. Would a hand buzzer be enough for that one?
Captain Moroni - rip off your t-shirt and wave it on a pole.
Or get an old time beard and go as an early prophet.
Who was the wife that Emma threatened to push down the stairs? Go all bandaged up.
Oh, oh! Top hat and a big cigar - Brigham after a talk on the Word of Wisdom (or was that Joseph what did that?)
A LD Saint who is prohibited from drinking caffeinated beverages.
An invisible LD Saint, because he's a Democrat.
A LD Saint with horns.
I knew a Southern Baptist preacher who believed the first and last myths, and I've met several people who believe all LDS are Republicans because the President of the church told all LDS to be Republicans.
Regarding horns, one of my wife's seven freshman suitemates entered college believing that anyone Jewish had honest-to-goodness horns. Amazingly, their mutual Jewish suitemate didn't have 'em. Amazingly, neither did my wife's neighbor and elementary and high school best friend, Ms. Heiman, when she came to visit.