They're running out of attention grabbers though. Port and Mana Drain are two of the only cards that are iconic, expensive, non-RL, and haven't been reprinted in a booster set.
So doing some quick breakdowns:
There's a total of 56 non-reserved cards that are over $30 in secondary market price today. Remove everything that was in MM3 or currently in Standard and that's down to 49. Remove cards from EMA and that's down to 45.
What's left mostly falls in a few categories: Portal cards (15), artifacts that missed MM3 due to Kaladesh Inventions (8), mid-era unreprinted Modern-type cards (6), cards that have risen from MMA/MM2 (6 that don't overlap another category), casual/commander cards (4), and never-reprinted-in-packs eternal staples (4).
Full list:
Capture of Jingzhou $400.50
Zodiac Dragon $222.46
Grim Tutor $204.70
Imperial Recruiter $191.27
Imperial Seal $160.17
Mana Drain $152.19
Rishadan Port $102.75
Riding the Dilu Horse $74.99
Flusterstorm $73.45
Dong Zhou, the Tyrant $64.99
Noble Hierarch $60.04
Horizon Canopy $58.79
Karn Liberated $57.39
Chalice of the Void $57.18
Crucible of Worlds $56.65
Doubling Season $54.95
Engineered Explosives $52.06
Mox Opal $50.37
Ancestral Vision $49.01
Ravages of War $48.05
Through the Breach $44.65
Leovold, Emissary of Trest $44.42
Goblin Settler $44.40
Dark Confidant $43.27
Zhang Fei, Fierce Warrior $42.28
Scapeshift $40.05
Grove of the Burnwillows $40.04
Three Visits $39.99
Food Chain $38.23
Yuan Shao, the Indecisive $37.99
Mishra's Bauble $37.36
Aether Vial $36.94
Emrakul, the Aeons Torn $36.49
Ensnaring Bridge $35.47
Arcbound Ravager $35.04
Temporal Manipulation $34.98
Sword of Fire and Ice $34.69
Meng Huo, Barbarian King $34.34
Sliver Legion $33.74
Kaalia of the Vast $33.66
Fulminator Mage $33.48
Pact of Negation $31.88
Xiahou Dun, the One-Eyed $31.84
Twilight Mire $31.14
Ugin, the Spirit Dragon $30.64
I bolded some that seem like they might be picks for this set.