Probably the easiest card to add is Revel in Riches , an enchantment which will provide you with an untapped Treasure token each time one of your opponents' creatures dies, and then win you the game if you have ten Treasures at the beginning of your upkeep.
Even very controlling 18-land decks can just be 18 lands plus several cards that draw or fetch lands for you (Cultivate, Field Trip, Pop Quiz with access to Environmental Sciences, etc.) and skip the 19th land
Firdoch Core Its not as impressive as Bender's Waterskin, but Firdoch Core at least offers an interesting spin on the 3-mana mana rock
Master Duel, the official digital app for the TCG, is currently running a 25th anniversary festival with login bonuses, free premium currency, and a special booster pack containing alternate art versions of Obelisk, Ra and Slifer.
Herald of Slaanesh is now an auto-include in any Demon decks, probably alongside Bloodcrusher too