In an interview with TV Line from Comic-Con, 24: Legacy showrunners Evan Katz and Manny Coto revealed they have plans for older characters to possibly make a return.
On returning characters
Evan Katz: We have plans, but right now our clear priority is these new characters, our actors, and having our audience build a relationship with them. But we have some surprises we have planned.
Manny Coto: We look at this as an extension of the 24 universe. This is not a reboot. So it’s an adventure that will be existing alongside, in-between the adventures that have already gone on. So that should kind of give you an idea of [where we’re going]. We may very well intersect with other characters that we’ve set up who are out in the world doing their own thing. Who are alive, obviously.