Part of the problem will be picking it up where the tv show left off. Fans of the tv show will want to know how Jack escaped New York, where he's been hiding out, what's happened to Chloe etc. While casual fans will just want a good 2 hour piece of escapism. Another problem is what to do with the real-time motif. Set it over 2 hours, like they did with the Redemption movie or set a 2 hour movie over 24 hours?
The going theory is that the first part of the movie would be 23 hours in "movie time" with the last hour of the movie happening in "real-time."