From cards printed in minor releases that never saw a release again, to cards that are exclusive to digital games , to Secret Lair Drops that have cards that have never seen a Universes Within printing
Along with letting players know how much value they should expect to open in a box on any given day, this will also allow for some interesting research on how prices (and expected value) change throughout a set's life
Turning Shrieking Drake into Dark Ritual and random tokens into Preordain takes over a game in no time
Aethergeode Miner : Creature (Dwarf Scout), casting cost of one generic and one white mana, 3/1
The idea here is to have a control shell with a lot of counterspells, some removal spells, and good value creatures to stabilize
When it becomes Monstrous, not only does it get +1/+ counters on it, but you also get to create X X/X tokens equal to the Monstrosity X value