So I'll openly admit that I'm running a mono black zombie deck (can be seen at http://deckbox.org/sets/133532), but I had a few thoughts.
First, I'm curious what type of playing the deck does the deck see? Casual? Or are we talking about possible competitive play? Are you willing to spend a bit to upgrade it?
Second, I'm in agreement with Thrifband about the Cemetery Reaper, but not as much about the Zombie Master. I've noticed the Zombie Master really doesn't end up helping that much, especially with Endless Ranks of the Undead which renders all the zombies expendable. To the extent that you're willing to spend to pick up a Death Baron or Lord of the Undead you get another +1/+1 effect with some added bonuses. Trust me when I say things get mean when you're generating 5-10 tokens a turn and they all have deathtouch.
I would drop the Reap the Seagraf in place of three Moan of the Unhallowed. Yes, the casting cost is one more and the flashback is two more, BUT, you get double the zombies. If you do the math, to get four zombie tokens out of the Reaps, you would need to spend 16 and need to cast, and flashback, two of them. Comparatively, the Moans will pull that off for only 11 and you only need one.
I'm not seeing much sense for the Preordain if there's only the one. Even if you had one more that would make it a more consistent and useful card.
Soulless One is a definite must have, as they become VERY big, VERY fast. I would replace the Unbreathing Horde with those if possible. Also Call to the Grave can be plain out sick as everyone else is dropping creatures while you're sitting there letting the horde grow. Pair that with Grimoire of the Dead (which you can discard zombies for as you'll get them back soon enough) and you have just one more NASTY combo that can easily decide a game (no matter how many opponents you're looking at).
I don't know that I'm really seeing the power of Nested Ghoul unless you can guarantee you'll be pumping him up or getting him hurt every turn. If you want to keep him in there you can imagine how a Prodigal Sorcerer or two (or something comparable) could prove VERY useful.
I like Diregraf Captain, but I think four might be too many, as they do have the three casting cost but require both colors to cast.
Lastly, you can get the same effect of Tragic Slip for the most part with Skinrender. I would even suggest changing the control cards to something more consistent like Go For The Throat, Terror, or Doom Blade.
Best of luck with it!