I think there should be 2 more films, one more set in the timeframe of the 4 previous films.
Then the final film could be a prequel to NOTLD, where we see 1960's society slowly turning into zombies.
I think that they should explain what happened and how the dead began to rise, maybe little flashbacks to what led up to the graveyard scene with Barbara in NOTLD.
Coolest line in that movie " They're coming to get you, Barbara". Its so funny!!