alright so quite a few cards i would cut

cellar door - its 3 mana for a chance to get a zombie not very reliable and quite slow.

doubling cube - weird card for your deck because unless you have huge mana reserves it will actually provide less mana overall for you.

ghoulcaller's bell - I guess it starts you off early but maybe it would be more reliable and more effective if you were to have think tank?

naturalize - is way too narrow in commander you would be better off with krosan grip if you are really worried about artifacts or enchantments otherwise use beast within to cover your bases.

lazav - you aren't really milling your opponent and since sidisi only mills you lazav seems itll just sit there on the field.

it that betrays - you don't have any sacrifice engine going on in your deck so you are purely leaning on him to do the heavy lifting and itll get killed before you can, just seems way too random for the deck.

duskmantle seer - as much as I like seeing someone cutting from the norm with sidisi decks so far you however have huge mana spells that can easily kill you avoid it or your lower your high end spells by a lot.

blightsteel colossus - not much in the way of mana ramp in your deck and although it shuffles back in your deck when you mill but does that alone justify a spot?

fabricate - way too narrow and since you are playing black you have much better tutors when you play black.

culling the weak - one time mana ramps in commander are never worth it unless you are a combo deck that plans to win that very turn.

Demonic Consultation - you dont have vizier in your deck which could of possibly made this card absurd in your deck otherwise itll hinder your graveyard strategy and it has 6 percent chance of making you lose the game outright.

tormods crypt - why not just replace a land with bojuka bog?

necromancers stockpile - if you are really dead set on cycling through your deck its fine but it will neither trigger sidisi ability nor will it create a zombie on its with the single zombie you have.

wreath of geists - not too many creatures in your deck and its a pretty vanilla card nighthowler would essentially be a strict upgrade over this.

as for additions it solely depends if you want to go down a tribal, aggro, midrange, or control route. If you can tell me that I can tell ya some suggestions.

I really like all your suggestions so far. As for where I want to go, I think midrange is what I want.

What about Oracle of Mul Daya Courser of Kruphix Garruk's Horde so you know exactly what's on top before you mill yourself?

Here's another spattering of ideas:

Deadbridge Chant Seems solid.

Whip of Erebos Seems deadly.

Chainer, Dementia Master Gross!

Hornet Queen Nasty!

Genesis This card's a gem!

Elvish Piper Because, why not?

Deathrite Shaman Solid 1 drop.

Treasure Cruise Nice.

Spiritmonger 'cause it's a beast!

Bow of Nylea Deathtouching Zombies!

Empty the Pits Lotsa Zombies!

Kamahl, Fist of Krosa Trampling Zombies!

Solemn Simulacrum Every deck needs one.

Glissa, the Traitor If you have artifacts, you can't go wrong with this back-stabber!