Claire didn't die on 9/11. Instead she and Mac realize they are missing something from their relationship. They realize just what is missing when Danny joins the lab. They decide to bring the young detective into their relationship. Protective Mac and Claire.
Catherine had a baby when she was a teenager; a boy, that she gave up for adoption. Many years later, she finds out the son she gave up grew up to be Greg Sanders. Bonus points if the biological father makes an appearance and is a crossover character. Leroy Jethro Gibbs from NCIS or David Rossi from Criminal Minds could be possibilities.