According to executive producer Howard Gordon while speaking at the ATX Television Festival, there is a possibility for Kiefer Sutherland to return as Jack Bauer during the second half of the first 12-episode season of 24: Legacy.
I do think the character has life in him, whether it's a movie or if he intersects with this new iteration of 24, [Legacy]. I sure would love to see him at some point on the show, and I don't think he's ruled it out, either.
And speaking of the last Bauer appearance in Live Another Day:
I don't know how public this is but we didn't question or consider this being Jack's last moment, Jack's end. We couldn't bring ourselves [to kill him].
Deadline is reporting that producers have an idea that could bring Bauer back in the last half of the first season to do some guest appearances that wouldn't conflict with Sutherland filming his new series Designated Survivor. There are also possibliities of other original cast members reprising their roles in guest spots.