Two cards that you might want to consider adding to this deck would be
cellar door and
nighthowler. Mill might not be the deck focus but it takes away from the opponent's resources and it gives you 2/2 zombie tokens whenever a creature hits the graveyard. This works well considering all the zombies you have. Also
Nighthowler would be good because it or any creature that it's bestowed upon gets stronger for EVERY creature in the grave. That is called an investment.