High complexity, lesser known cards. Rewarding Magic historians and those who can spot the key interactions.
The following can be considered build around or signals to move into a particular archetype.

Snow (all basics are snow)
Dead of Winter, Skred, Blessing of Frost, Ascendant Spirit, Reidane, God of the Worthy
Aura Interaction
Armored Skyhunter, Zur the Enchanter, Hateful Eidolon, Sram, Senior Edificer
Constellation
Strength from the Fallen, Underworld Coinsmith
Lantern Control
Sage of Mysteries, Lantern of Insight, Codex Shredder, Precognition, Thoughtpicker Witch
Monocolored
Benalish Marshal, Inundate, Ayara, First of Locthwain, Jaya Ballard, Old-growth Troll

Featuring:
Adventure
Morph
Flash
Clash
Sagas
MDFCs
Mill
Lesson/Learn
Vehicles
Escape
Mutate
Bestow
Delve
Constellation
Strive
Demonstrate
Phasing
Afflict
Ninjutsu
Raid
Parley
Conspiracy Draft Mechanics