First, since Kelly said in an interview that her favorite composer was Jerome Kern, and I've never heard her do anything he wrote, she should sing some Kern. But not "All the Things You Are," because every female vocalist has recorded that song, most recently Kristin Chenoweth. Maybe another ballad almost as good, like "Long Ago and Far Away," or "Look for the Silver Lining" or the more upbeat "Who."
Then I would love to see her perform that tour de force from her latest album, "They Don't Let You in the Opera (If You're a Country Star)." It's hysterical enough just hearing her sing the song, and if she added some physical humor to the performance it would bring down the house.