Domestic and international marketing demands drive movie studio decisions on which movies to make. Choices are based almost exclusively on "high-concept" ideas. High concepts are movies designed to break the mold in some essential way and whose main thrust can, as Steven Spielberg says, hook someone in "twenty-five words or less."
The zombie genre, because of its inherently radical nature, is particularly sensitive to high-concept movie making, so any successful zombie film idea must first address this story hurdle. The movie "28 Days Later" did this by focusing on people becoming fast-moving zombies four weeks after a virus spreads.
Lengthy and broad, the zombie movie genre has been explored and exploited from just about every angle. That makes creating new zombie film ideas challenging. One significant way to generate new ideas is through studying the established form, then figuring-out a way to twist it in an innovative manner. "Sean of the Dead" did this by poking fun at the genre's many cliches.
The momentum of a zombie movie's plot usually comes from the hero's struggle to survive and overcome seemingly insurmountable odds. Often, the rules of that struggle -- what the zombies can and can't do, for example -- offer a prime area for twisting the genre. Equally, the how and why of the high concept itself should offer the opportunity for a twist. For example, zombies that fall in love might also, because they have learned emotions, suddenly feel jealousy and turn upon themselves.
Establishing a consistent reality within a particular zombie movie is essential. Creating a structure for the unique world you have formed will allow what would otherwise be an average zombie idea mushroom into an innovative picture that can maintain itself. For example, a dark zombie-filled tragedy suddenly turning into a light musical wouldn't be a cohesive movie idea.