mt1: Lotus, Scav
at1: land
mt2: Lotus, Yas, attack (m@20, a@17)
at2: Mayor1
mt3: Lotus, Take Mayor, attack (m@20, a@9)

If you play Mayor2:
at3: Mayor 2, at (m@19, a@9)
mt4: Lotus, Take Mayor, at (m@19, a@1)
at4: pass
I now have more attacker than you have blockers...Mayors can't flip in time because I cast Lotus every turn. Your only other option is to not attack on turn 3, but rather chump Scav with Mayor. Then next turn I steal Mayor anyways and kill you.

If you don't play Mayor 2, your Mayor flips but you don't get a wolf token because it flips on my upkeep and you just die on the swing.

at1: land
pt1: Lotus, Scav
at2: Mayor1
pt2: Lotus, Yas, attack (m@20, a@17)
at3: Mayor2
pt3: Lotus, Take Mayor, attack (m@20, a@8)
at4: pass
pt3: Mayors transform into 4/4's

At this point, it's two 4/4's (which Yasova can't steal) versus a 4/2 and a 3/3. POSValkir has no way to get through.

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
Mana Echoes: Whenever colored mana is added to a player's mana pool, that player adds an equal amount of colorless mana to their mana pool.
Bearify: All creatures have base power and toughness 2/2.
Night of Thorns: All creatures get -1/-1. Noncreature spells cost more to cast.
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.