Open The Floodgates
Its Shorthand rhymes with peek-a-boo.
There Can Be Only One
This in-archetype senet switch on legs stands as tall as the sun once it hits the field, let's just hope your opponent has some cards on their side of the board.
Mekk-Knight Indigo Eclipse
"This card is a good choice for a mill Deck because you can see when your opponent has a card you don't want them to play, and when they have a card you should avoid milling, like 'Revival Golem'."
Convulsion of Nature
Cards sent to the GY are banished instead.
Marco Cosmos/Dimension Shifter
The amount of cards the going first player has in hand during the standby phase if the first turn.
6
This Archetype is chocked full of its share of floodgates, as long as you have enough monsters to tribute for their maintenance costs.
Amorphage
The Vaylantz Card that searches Senet Switch.
Vaylantz Wars - The Place of Beginning
"'Yubel' can be Special Summoned during the opponent's turn with this card, and during the next turn, it can subsequently changed to Defense Position to easily Special Summon 'Yubel - Terror Incarnate'."
Limit Reverse
'Clears the board', but leaves your opponent with (at least) 1 card.
Evenly Matched/Nibiru The Primal Being
Believe it or not, Pot of Greed has somewhat fallen off in play in goat format, with some duelists preferring to play this draw card instead. Although a lot of players run both.
Graceful Charity
This floodgate found its home in weather painters- wait that can't be right *ahem*
...in Floowandereeze for its synergy and well as making the deck even more annoying to come up against.
Dimensional Fissure
This card is most commonly used by giving it to your opponent, to enable infinite loops with its column destroying capabilities.
Yajiro Invader
"This card is useful for Ritual Summoning, since all Ritual Monsters are only up to Level 8. Especially when the Ritual Spell Card requires exact Levels such as 'Dawn of the Herald', 'End of the World' and 'Contract with the Abyss'."
Gagaga Magician
Negates an attack and ends the battle phase.
Negate Attack/Swift Scarecrow
You've just tributed summoned for 'Summoned Skull' and it looks like you have the advantage, but oh no! Your opponent normal summons this card, and now your monster is going on a vacation to the different dimension.
D.D. Warrior Lady
These two monsters found themselves on the banlist for their attribute related floodgates, one being an extra deck monster and the other being an easily searchable main deck monster in the right decks.
True King of All Calamities and Archnemeses Protos
The Magical Musketeers have effects that activate if a Spell/Trap is activated in their column, and there's some pretty good effects. This card even lets you special summon from deck!
Magical Musketeer Starfire
"'Infinite Cards' can increase the amount of damage inflicted by this card's effect, particularly since it also removes the hand-size limit of your opponent as well; more cards to discard means more damage is inflicted."
Chaos Emperor Dragon - Envoy of the End
Target and negates an opponents effect monster.
Infinite Impermanence/Effect Veiler
You activate the effect of 'Relinquished' to equip one of your opponents mosters but they chain 'Raigeki Break' to destroy your 'Relinquished'. The chain resolves but what gives!? Why has this what happened to their monster?
Nothing
This floodgate has a rather simple effect, yet is the bane of Synchro, Xyz and Ritual Decks.
Stygian Dirge
The maximum amount of cards you can destroy with the effect of Springans Ship - Exblowrer targeting an extra monster zone.
"This card can be Tribute Summoned with one Tribute with 'Unshaven Angler' and The 'First Monarch'"
Mare Mare
Prevents Effect Damage.
Pikeru's Circle of Enchantment/Hanewata
This goat card has had 5 erratas in its lifetime... all pretty much just updating its wording as Yu-Gi-Oh progressed.
Scapegoat