I'm back *******! Okay, so right now I think Necrovalley is generally a better floodgate than Zombie World, but Zombie World is also pretty good rn, and will be better very soon with the release of Flunders, since you're essentially running 11 Mask of Restrict in archetype that also still hates on other meta strategies. Zombies is a nice shell for control, because you have built-in recursion and the ability to take incredible advantage of Rivalry of Warlords in a way no other deck can, and Super Poly, again in a way few decks can.
So, for essential Zombie cards, I'm going pretty hard on Zombie World. There are a whole 3 copies and Terrraforming, which is obligatory. Necroworld Banshee is a 3 of in my opinion, so want to be able to normal summon this in a pinch, you want to send it from your hand with Uni-Zombie, and most importantly you want a Zombie monster you can activate at spell-speed 2 for Balerdroch that doesn't go to far out of your way. That last part makes it worth running 3, especially since the deck is pretty reliant on Zombie World to get its game on. 2 Glow-Up Bloom, occasionally seeing these is good, these are easy access to Balerdroch or Golden Lord, usually Balerdroch since you'll have Zombie World on the field. 1 Mezuki, this is just a nice little bit of recursion that also activates Balerdroch, admittedly a little slow but still useful to have another way to activate Balerdroch that nets you value. We have 2 Shiranui Solitaire and 3 Uni-Zombie. You'd always rather just have the zombie than a searcher, and you only really want to see 1 of these, but you do want to see 1 of these, so 5 is the perfect number. Trust me, you think six is a good idea, and then you clog Uni-Zombies. 2 Balerdroch, you have lots of easy access to this in the deck, so you only need 2. 2 Golden Lord. The Special Summon from the hand while returning to the hand is actually relevant in this deck, but really it is here for a spot of in-engine removal. Still, it is a great beatstick and it activates Balerdroch while non targeting sending something to the GY, I'm seeing non-Zombie decks play this and if you were really looking to break boards you could even run 3, but my gut tells me 2 is the right number. 1 Vampire Fraulein. If you're new to Zombies you might wonder why people play this card, it is basically an OTK enabler, helps close out games while being a battle-phase hand trap. Yeah, the card isn't that pushed, but pushing in for an extra 3000 damage is too good to pass up. You only need 1, we don't want to see this all the time, we just want it around. Super Poly is basically a Zombie card, you use this to summon Dragonecro with 2 of your opponent's monsters, which is pushed. Rivalry is also basically a Zombie card since it combined with Zombie world locks people out of summoning non-zombie monsters if they control a monster, a powerful "combo". 3 Pot of Extravagence, I think this deck needs raw card advantage more than consistency, and it doesn't care about its extra deck that much. 1 Foolish Burial, this is your last copy of Zombie World but also more access to Balerdroch and Mezuki, as well as sending Glow-Up Bloom for Balerdroch. That left room for a reasonable suite of generic interruption. Droplets is a staple for a reason, and we're a GY deck, so 3 of these seem really obligatory. Evenly Matched seems pretty good right now with Tri being one of the best decks, if not the best deck, in the format, and it handles a lot of the midrange decks that are more powerful in the meta right now. It is also just a card with a proven pedigree, in Zombie decks in particular, so I wanted 2 in the main. That left 4 cards. I like the power of the Solemn Strike/Torrential Tribute "Combo" wherein they need a countertrap to save their board - we have the tools to deal with the creatures protected from these cards, so this seems reasonable. It also works in conjunction with Evenly Matched to clear away Backrow and deal with monsters, I mean, it just seems solid, solid enough that in some matchups I may want the third copy of any of these cards, or all of them, and have them in the side.
You need Linkuriboh for Glow-Up Bloom and we run Extravagence so we have 3 Linkuriboh. Avendread Saviour and Vampire Sucker are cards you go into regularly, and you may even want multiple Suckers in a game, so we're for sure on 3 of each of these in an Extravagence deck. Verte Anaconda could come up some time, but I'm fine running 1. Dragonecro, on the other hand, is a card you might make multiple copies of in a game if you don't banish it, a clear 3 of. Starving Venom is obligatory in Super Poly deck, but I also like Triphoverytium (or whatever, the 3 darks monster) because it is actually common enough to make 3 darks, or to make 2 darks and for me to be able to make Verte Anaconda, send Super Poly.
Side deck. First few cards are for backrow matchups. Shared Ride is for going first anywhere Droll is good. Different Dimension Ground is essentially a turn skip, and these 2 cards overlap in a lot of matchups. When you're going second you instead go for board breaking cards and backrow removal as necessary. Torrential and Solemn Strike contribute to these plans, being cards that are good going second as well as going first. Solemn Judgement is your anti-backrow removal card - you basically should be running this game 2 to 3 to protect your traps and your Zombie World from the influx of backrow removal.
I decided to run Ice Dragon's Prison over D D G, since with Zombie World on the field its grave hate that is also a huge blow-out, a little more reactive rather than being a temporary floodgate, but IMO still often a turn-stopper, and useful in the same matchups as D D G thanks to Zombie World.
So, I decided Evenly was a little weak going first in this deck, so it became the backrow removal in the side to focus on the more flexible TT + SS combo. That opened up some space in the side deck - I already has Ice Dragon's Prison as a GY hate, normally restricted to use against Tribal deck but really good in a Zombie World deck as generic GY hate. I also already had Shared Ride, to put the hurt on searching, and Solemn Judgement to shut down to backrow removal on a flexible card that answers any threat that would hit the table. The deck is good at putting up a couple high-quality interruptions, and Zombie World and Rivalry of Warlords gives us a low of floodgates. Between our answers, Drytron and Tri-Bri are covered really well, and so is Rokkets. Virtual World and Prank Kids is my next concern. I feel like Virtual World is covered pretty well by Shared Ride and Evenly Matched along with board breakers. That leaves Prank Kids as the matchup that needs the most coverage. I felt like having some Twin Twisters to go with the Evenly Matched would improve this matchup, while generically being useful against the midrange and fast control decks that are popping up as rogue decks right now. That left one space for one card, and I decided to run a lone copy of DRNM, so we can play games going second. If we only have space for a 1 of, I want it to be impactful when I do see it, because it won't show up enough to be the swing in the matchup on its own, and I like it as the redundant "4th" copy of Droplet.