They are going to make a movie aren't they? So i guess a dead Jack won't work well -- but they could set the movie before Jack was kidnapped to China???
There are gaps between seasons when they could set a 24 movie. But there are a few issues with making such a movie:
1) The storylines have progressed to the point where it would be confusing even to longtime fans if the story jumped backwards to fill in the overall story arc. Better to make it part of the overall history continuing from the previous season, then have the next season's events occur after the movie. X-Files did this, IIRC.
2) By definition, the 24-hour part doesn't exactly work for a movie. Would it be a real-time 2-3 hour movie? Would it be portions of a 24-hour period? Something's gotta give there.
3) The production schedule of one season of 24 is spread out over about ten months, including writing, preproduction, shooting, editing and post. It's pretty well-known that they don't finish writing the last few episodes until after the early episodes have started airing. So when would they have time to shoot a movie? And if it's part of the big 24 story arc like in 2), then they'd have to write the movie and at least parts of the preceding and following seasons way, way ahead of time, much farther in advance than they ever have before.
I'm not saying they shouldn't make a movie or that it wouldn't be good, just that there are some issues that would have to be ironed out. Here's one idea: make the movie the end of the series, with the final season ending on a cliffhanger, and the last two hours of action released as a feature film instead. But what do I know?