Either Blightspeaker or Wheel of Fortune.
For Blightspeaker, I like the art, creature typing, the flavour text and the abilities present on the card. Everything is just so right, and is probably favourite creature. It also probably helps that my two favourite tribes are Rebels and Clerics.
Wheel of Fortune is a little different. It is one of the first cards I remember having, and is one of the few that I still have my original copy of. Every time I play my copy, currently in my Nin, the Pain Artist commander deck, I feel taken back to my early days of playing MTG with friends during School at Lunch.
Notorious Throng is legitimately one of the best cards in the deck for Tetsuko. The amount of power and board presence that it gives you is unparalleled for the deck.
I would probably also look at Muddle the Mixture as a way to grab a quick infect creature, or removal in a pinch. You have a lot of CMC 2, so it seems pretty good.
A card I've been looking at myself for Tetsuko, which is dirt cheap I might add, is Turbulent Dreams. I could see this as being a budget replacement for Cyclonic Rift in your deck. While lacking the instant speed, with the low cost and the amount of cards a Tetsuko deck can draw this could be a good way to use even the worst cards in your hand at any moment.
I think the only other card I might consider would be Corrupted Conscience. Not only is it a way to steal an opponent's creature, and give it infect, but it can also give one of your own creatures infect as well.
Overall, I really like what you've done with the deck, if you have anything more to say I'm always happy to hear peoples experiences with Tetsuko decks. Hopefully, I hear back from you soon.
Blade is a card that I've always considered, but my main concern is the amount of aggression it can defer onto you, especially with infect. Whilst I think this deck can do well in a multiplayer setting attacking everyone equally is not my general gameplan. What sources of Infect and Proliferate are you running? I might even look at the War of the Spark card Guildpact Informant from the Jace Planeswalker deck to further my means in a deck like that. In addition, could I see your decklist pretty please.
Creature-Human Knight
Flying
When ~ attacks, create a 1/1 white soldier tapped and attacking
1/2
Source: https://twitter.com/magicmadridyt/status/1143100055819366400?s=21
Artifact-Equipment
Equipped creature gets +10/+10 and loses flying.
Equip 8
Source: https://www.pcgamesn.com/magic-the-gathering-arena/core-set-2020-colossus-hammer
Creature-Nightmare
Whenever a swamp enters the battlefield under your control choose one-
3/3
Source: https://twitter.com/crokeyz/status/1140605527648624640
So with Modern Horizons fully out in the open I've been meaning to make a long overdue update (I didn't update with War of the Spark since that set was mediocrity incarnate when looking for cards for this deck). Overall, I think Modern Horizons gives us a few interesting options for the deck, and I'm more than a little excited for testing out these cards.
This is just some generalized deck updates to help smooth out the deck. Whilst there aren't too many, I do have some future ones in the back of my mind, but I just need to acquire the cardboard first.
Creature-Vampire
Whenever ~ or another creature dies, put a +1/+1 counter on each vampire creature you control.
1/1
Source: https://www.twitch.tv/robbieyoon