A prequel would be nice, but, I don't think a movie could do our favorite period justice. There's simply too much meat to fit on the sandwich.
Maybe they could focus on the events surrounding the Catilinarian Conspiracy--it provides a nice foreboding of conflicts to come, with all the major players still on hand (Caesar, Servilia, Pompey, Cicero, Cato, etc).
I kinda think my namesake would provide veritable feast of scandal, lust, violence and hypocrisies to appease any 21st century audience!