Brawn

Mind Slash is the Thoughtseize version of Attrition except you can even take lands

Bridge from Below

Aqueous Form or something to make Sidisi unblockable?

Prized Amalgam

Thanks for the comment Mandalorian!

In commander I think I'd prefer Sadistic Hypnotist over Mind Slash since the sac is free, Hypnotist is more easily recurable, and I feel like the targeted discard is not quite as good as just knocking out 2 cards (I could be wrong about that thought).

I do like Bridge from Below, and unblockable for Sidisi has crossed my mind, gonna have to give some more thought to those ones.

I'm not really concerned with giving my dudes trample, the idea is more go wide and make them hard to block (deathtouch and/or flying). Filth or Zombie Master seem a little more helpful here. On the flip side though, I am in the market for a Craterhoof Behemoth as a finisher, but that card is a little on the expensive side.

Prized Amalgam was in here, but I wound up cutting him since all he does is give a 3/3 body for free, something I'm almost getting everytime I mill myself.

Rooftop Storm is perfect for zombie decks with Blue in them. Infinite loops for days. Example: Havengul Lich + Rooftop Storm + Sidisi, Undead Vizier to haul every single zombie out of your library for 1 colourless mana each using Sidisi's self-exploit. Great for loading your deck with OP zombies that aren't practical to play due to high CMC, which you then ignore.

Thanks for the comment crossbones.

I like Rooftop Storm a little better in a more singularly focused zombie build. I used to have it in here, but with all the non-zombies floating around for milling purposes I found it frequently wound up doing nothing for the vast majority of hands that I pull. The card does go off with the aforementioned cards, as well as Gravecrawler and any sac outlet that produces value like Grimgrin, Corpse-Born, Carrion Feeder, or Diregraf Colossus, but the deck is about abusing the ability to squeeze as many tokens out of Sidisi as humanly possible and most of the cards that facilitate that do not benefit from Rooftop Storm.

Necromaster Dragon or Ruthless Deathfang perhaps?