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

Deckbuilding
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 deck as your library along with your normal starting hand. (Expect additional bans.)
Storm Scale: Every deck must lose 0-6 against Island, Island, Storm Crow.
3 Fold: Submit a 3 card deck. You start the game with three copies of each of those cards in your hand. You have no maximum hand size.

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.
Auras!: For each permanent card in your deck, you may choose an aura with the same mana cost as that card. That card always enters the battlefield with a token copy of that aura attached to it.

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

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

Deckbuilding
Cats!: Your deck must contain at least one Cat. (Changelings count.)
3 Fold: Submit a 3 card deck. You start the game with three copies of each of those cards in your hand. You have no maximum hand size.
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 deck as your library along with your normal 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.
Auras!: For each permanent card in your deck, you may choose an aura with the same mana cost as that card. That card always enters the battlefield with a token copy of that aura attached to it.