While normally you calculate a spell's mana value by looking at its mana cost, Edge of Eternities adds an interesting wrinkle: warp
Floodborn characters born from the game fictional concept of a "cataclysmic ink flood": an event unleashed when a forbidden book was opened, changing heroes and villains alike into more powerful forms, though many magical "pieces of lore" were lost in the ink flood as well
One of the newest cards on this list, Serra Paragon was introduced in Dominaria United, and is an interesting example of a common debate in Magics rules: should the rules be interpreted as-written, or as-intended
Darkpact (Alpha, Beta, Unlimited, Revised) The classic ante switch
It was a creature whose power and toughness were equal to the number of creatures you had in play (okay, technically non-Wall creatures)