Bontu's Last Reckoning: Our backup sweeper for
Damn, which we can slam down a turn earlier. While keeping the lands tapped is a serious drawback, being able to cast the sweeper with one less land is invaluable, especially because the deck can sometimes get stuck on 3 lands due to
Smallpox. Generally bring this in against any sort of aggro or creature heavy deck.
Dauthi Voidwalker: One-sided grave hate on legs that helps with the rack plan by ticking away at our opponent's life total with a 3/2 evasive body, just like Grief. An alternative would be to run 3-4 Leyline of the Void, which we can pitch to Grief if we draw it later. While there are definitely some graveyard shenanigans going on in the format with Dragon's Rage Channeler, Murktide Regent, and Living End, there aren't too many extremely graveyard heavy decks we need to hose like dredge (besides Living End maybe) which is why I'm not running the Leylines.
Delirium Skeins: Bring this in against any opponent you think will board in Leyline of Sanctity; e.g. some white decks but definitely bogles and enchantress. Also, I generally leave in Grief but take out all 4 IoKs for other cards. We want to minimize the number of dead draws we have which is why I put in basically any removal or Tourach's instead of targeted discard.
Kaya's Guile: Plays double duty as backup grave hate and additional removal. The 1/1 spirit mode is very relevant, while the gain life one can sometimes win you the game against burn. Can be decent against the Indomitable Creativity deck.
Pithing Needle: You can name planeswalkers, Colossus Hammer, Stoneforge Mystic, Expedition Map, Mishra's Bauble and all sorts of other troublesome permanents with activated abilities.
Plague Engineer: Obviously good against any tribal deck like elementals (bye Risen Reef), especially hoses infect now with the "phyrexian" creature type errata.
Stony Silence: More HammerTime hate, hardened scales affinity and classic affinity are both also a thing again so there's that. Incidentally shuts down Bauble but I don't recommend bringing it in against bauble decks. It is okay against tron, but better when we're on the play which we usually aren't -- bringing it in here is a judgement call.
Surgical Extraction: Graveyard/combo hate which works well with all the discard we are running. If we Grief + Ephemerate turn 1 and the opponent has doubles of a card in their hand, we can add insult to injury by using this to take out 4 of their cards.
Tourach, Dread Cantor: A must-remove threat and discard 2-in-1 package generally good in the slower matchups, but frequently too slow for fast decks like hammertime and burn. He's obviously good against most white decks, but he really shines versus control decks. If you don't have the fourth land, it's not always necessary to cast him kicked, although it always feels like a waste.
Vindicate: Our final piece of catch-all removal for any problematic permanents, whether that's a creature, a Chalice of the Void, Teferi, Time Raveler, Wrenn and Six, or a tron land. Since it's a good card in basically every matchup, I usually put it in for IoKs against decks running Leyline.