Cards that let the table vote for different effects on resolution are extremely interesting

A Killer Among Us Adaptive Automaton An-Zerrin Ruins And They Shall Know No Fear Arachne, Psionic Weaver Arcane Adaptation Ashes of the Fallen Banner of Kinship Barbarian Guides Belbe's Portal Black Market Connections Blood Oath Bloodline Bidding Bloodline Pretender Bloodline Shaman Brass Herald Caller of the Hunt Callous Oppressor Cavern of Souls Celestial Reunion Chronicle of Victory Circle of Solace Coalition Flag Collective Inferno Conspiracy Coordinated Barrage Cover of Darkness Crippling Fear Deification Distant Melody Doom Cannon Door of Destinies Elvish Soultiller Engineered Plague Etchings of the Chosen Fertile Imagination For the Ancestors From the Rubble Gathering Stone Gauntlets of Chaos Giant Slug Harmonized Crescendo Haunting Voyage Herald's Horn Icon of Ancestry Illusionary Presence Imagecrafter Instruments of War Juxtapose Kindred Boon Kindred Charge Kindred Discovery Kindred Dominance Kindred Summons Leyline of Transformation Lifecraft Engine Littjara Luminescent Rain March from Velis Vel Metallic Mimic Mirror Entity Avatar Mirror of the Forebears Mistform Mutant Mistform Wakecaster Molten Echoes Morophon, the Boundless Obelisk of Urd Oko, Shadowmoor Scion Outbreak Pack's Disdain Pact of the Serpent

Start with the amount of damage being dealt to the creature and determine what is "lethal." This is usually the creature's toughness minus the amount of damage that it already has marked on it, but ignoring any replacement or prevention effects that will modify this damage
When two good cards in their own right have a powerful synergy then you gain loads of free added value to your deck
A single Underground Sea costs more than that spec, and the financiers investing in duals are buying lots of dual lands
Bloodletter of Aclazotz Bloodletter of Aclazotz has carved out a niche for itself in some Constructed formats, especially with its partner in crime, Unstoppable Slasher