Its true that Ms. Palin graduated from the University of Idaho School of Journalism, but there isn't much else that qualifies her for the $20 bill. From Wikipedia, she won a beauty contest for Ms. Wasilla once, she served for a decade or so on the Wasilla City Council. She was elected governor of Alaska in 2006. During her term in governor, she only had one scandal, in 2008 she fired a guy who had fired her brother in law earlier. In 2009 she quit halfway into her term as governor. As governor, she pardoned several turkeys, but no humans.
Since that time, Ms. Palin has mostly worked as a reporter/public speaker.
Ms. Palin might be a fine American and she does have some memorable quotes but 1. She's not dead yet and 2. She hasn't exercised broad influence on the history of the United States (yet).
Play it safe. Go with MLK or Reagan.