Rather than see 24 disappear completely, my idea is to launch a new show simply called "CTU". The new show would differ from "24" in a lot of ways. First off, I would ditch the 24 hour realtime format. It was great for establishing a unique identity, but I think it hurt the show in some instances where the main plot was finished at around 16-18 hours and we got unnecessary twists fill the remaining hours. This would allow the show to take a more long term approach to the characters and to delve into their personal lives more. You could have a mix of serial episodes with related plots and self contained episodes.
Basically, I'm saying make 24 a lot like the british show "MI-5" minus the left wing political slant.