Any card banned in Vintage.
Any deck that can win before a blank opponent can take a full turn. (A blank opponent is an opponent with three blank cards.)
Any deck that can reduce the number of cards in a blank opponent's hand by more than one card during a single turn.
Barren Glory
Blackmail
Cabal Therapy
Chancellor of the Annex
Channel
Dark Depths
Ensnaring Bridge
Flash
Foil
Force of Will
Ghost Quarter
Grave Titan
Inquisition of Kozilek
Laboratory Maniac
Leyline of Anticipation
Phyrexian Revoker
Shelldock Isle
Show and Tell
Shrieking Affliction
Strip Mine
The Rack
Thoughtseize
Wasteland

d1 Goes First
d1: Forest, Llanowar Mentor
d9: Mutavault, Mox Sapphire, Dragon's Eye Savants
d1: pass
d9: attack for 8 (d1 at 12), d1 makes Llanowar Elf #1
d1: Llanowar Elf taps to become a 2/2
d9: attack for 8 (d1 at 4), d1 makes Elf #2
d1: tap Elf #2 to become a 2/2
d9: attack, d1 trades Elf1 with Mutavault and makes a new Elf to chump block the 6/6
At this point, d1 can chump block indefinitely while making Elf2 arbitrarily large, eventually winning.

d9 Goes First
d9: Mutavault, Mox Sapphire, Dragon's Eye Savants
d1: Forest, Llanowar Mentor
d9: attack for 8 (d1 at 12)
d1: pass
d9: attack for 8 (d1 at 4), d1 makes Elf1
d1: tap Elf1 to become a 2/2
d9: attack, d1 makes a chump blocker and takes 2 (d1 at 2)
d1: pass
d9: attack, d1 trades Elf1 with Mutavault and makes a chump blocker
At this point, d1 can chump block indefinitely, but has no way to win.

9t1: Mox, Vault, Savants
5t1: Golem1
9t2: At with Savant (9@20, 5@14)
5t2: Golem2

If attack with Golem1: 9 no blocks (9@15, 5@14)
9t3: At with Savant (9@15, 5@8)
From here if 5 attacks with both, 9 lets it through and goes to 5, winning on the swing back with both Savants and Vault for exact.
If 5 attacks with 1, 9 swings with Savants and can block one Golem with Vault allowing them to win on the next turn.

If no attack with Golem1: pass
9t3: At with Savants...if 5 double blocks, Savant kills both Golems leaving Vault to win. If 5 single blocks, they can't race, and if 5 doesn't block they can't race.

Formats at the top are the most likely to be removed if we end up with more than five.

Deckbuilding
Storage Wars: Your deck must contain at least one storage land.
Cats!: Your deck must contain at least one Cat. (Changelings count.)
Proto Gear: Can only use cards that have seen print in the first set of an expansion block. Core sets and stand-alone sets are not expansion blocks.
Collector: You start the game with a countably finite quantity of copies of one of the cards in your starting hand as your library along with your starting hand. (Expect additional bans.)

Gameplay
Night of Thorns: All creatures get -1/-1. Noncreature spells cost more to cast.
Bearify: All creatures have base power and toughness 2/2.
Community Effort: All spells and abilities have convoke. Noncreature spells that require a mana payment must be paid for using only mana produced by creatures and/or by tapping creatures through convoke.
But Wait, There's More: Hand: First 2 cards. Library: 60 copies of 3rd card. Play one land from outside the game each turn. Milling works. Additional bans.