Two topics that won't come up even though both candidates will veer to domestic topics because it's their last chance to make any significant change in the race are

Gun Control
Environment in any meaningful capacity

Issues Around the Last Presidential Debate: Different Viewpoints
http://www.washingtonpost.com/blogs/ezra-klein/wp/2012/10/22/wonkbook-the-last-presidential-debate-of-2012/