A signpost uncommon is a two-colored card that acts as a guide for limited archetypes in a set. For example, Risen Reef is a signpost uncommon for GU to be Midrange Elementals, or Heroic Reinforcements is a signpost sorcery for RW tokens.
Then I would design a UB creature that helps define a mill archetype in a limited format:
Memory Thief
Creature - Faerie Wizard
Flying
Whenever Memory Thief deals combat damage to a player, that player puts that many cards from the top of their library into their graveyard.
Whenever a land card is put into an opponent's graveyard from their library, draw a card.
1/3
Then I put the brief for the next poster to fulfill.
Design a creature for a blink archetype.