T.A.P.P.E.R.

{T}, Tap an untapped creature you control that has a hat: Tap target creature.

Haberthrasher

Deathtouch
Whenever another creature you control that has a hat enters or a hat sticker is put on a creature you control, each opponent loses 1 life.

Hat Trick

Target blocking or blocked creature you control gets +1/+1 and gains first strike until end of turn. If it has a hat, it gains double strike until end of turn. If you and that creature each have a hat, the creature gains triple strike until end of turn.

"Brims" Barone, Midway Mobster

When "Brims" Barone, Midway Mobster enters, put a +1/+1 counter on each other creature you control that has a hat.
"Brims" Barone, Midway Mobster has menace as long as you're wearing a hat.

Park Re-Entry

Return up to two target creature cards that each have a hat and/or mana value 3 or less from your graveyard to the battlefield.

Starlight Spectacular

Parade! — At the beginning of combat on your turn, choose creatures you control one at a time until each creature you control has been chosen. Each of those creatures gets +1/+1 until end of turn for each creature chosen before it. (Places everyone! The first creature in line gets +0/+0.)

Rat in the Hat

{T}, Sacrifice Rat in the Hat: Until end of turn, you may cast target creature card that has a hat from your graveyard.