So I had an idea that was inspired by Hearthstone with the "secret" cards that don't take effect until something happens. This is similar to that, though with how YGO works, they ended up having to work quite differently. Still, I think I made it work as well as I possibly could, though suggestions and questions are welcome. You can also find these on Duel Portal under the set MASK.
What are Mask Monsters and how do they work?:
Mask Monsters are a form of Main Deck monsters, they cannot be Ritual or Pendulum Monsters. Mask Monsters can be Normal Summoned or Set, you can choose whether to mask them when you Normal/Tribute Summon them, and if they are Set, you choose when you Flip Summon them. Masking is not a form of Summoning. If they are Special Summoned, you cannot mask them, with the exception of cards that state otherwise, such as Mask Entrance. You cannot mask a monster if their Normal Summon is under a certain condition that makes them able to perform an extra Normal Summon of a card in an Archetype or if the monster itself has a restriction on Normal Summoning, such as Ghostrick Kuchisake-onna. When a monster is masked, they are placed face-down instead of face-up, however they are still treated as face-up monsters, and can attack, be attacked, change battle position or be targeted by effects when appropriate.
What it means to be masked and how to become unmasked:
Mask Monsters almost always have an effect that unmasks them, making them become face-up, though other effects can also unmask them, such as Unmasking. If a Mask Monster's effects are negated, they are unmasked. If a Mask Monster battles, at the same time in the Damage Step that face-down monsters are flipped face-up, they become unmasked. When a monster is unmasked, an effect may become available on activate. Some cards can even mask themselves, even if they weren't masked before, such as Mask Switcher. You can still use masked monsters for Synchro, Xyz Summon etc. and if they are attached to Xyz Monsters, you attach them as normal, though this is not considered unmasking them. Masked Monsters retain their ATK/DEF that have gained or lost if they become masked or unmasked, though cards such as Equip Spells may be destroyed by game mechanics if they are not appropriate targets anymore. You cannot unmask a monster if that monster cannot be on the field because of a card preventing it from doing so for various reasons, such as Level or Attribute restrictions. When a masked monster activates its effect that would unmask it, you show the card to your opponent so they can confirm this, as they would not be visible otherwise. This is the only legal time you can show them to your opponent.
If a monster you control is unmasked (except during the Damage Step): You can Special Summon this card from your hand. If this card is targeted for an attack: You can unmask this card, then target 1 other Mask Monster you control; change the attack to that target.
If this card was unmasked during the Damage Step, and is destroyed by battle during that Damage Step: You can add 1 Mask Monster from your Deck to your hand, except "Anonymous Actor". You can only use this effect of "Anonymous Actor" once per turn.
When you take battle or effect damage: You can unmask this card, and if you do, gain LP equal to the damage you took, then if you gain 1000 or more LP, you can Special Summon 1 "Lucky Sprite" from your hand or Deck and mask it.
When your opponent Special Summons exactly 1 monster from their Extra Deck: You can unmask this card, and if you do, negate that monster's effects, also it cannot attack.
When your opponent's monster on the field activates an effect (Quick Effect): You can unmask this card, and if you do, negate that effect, and if you do, that monster loses 500 ATK and DEF. Once per turn, if this card destroys an opponent's monster by battle: You can target 1 Mask Monster in your GY; add it to your hand.
Once per turn (Quick Effect): You can mask this card. If another Mask Monster you control is unmasked: You can unmask this card, and if you do, it gains 500 ATK and DEF until the end of this turn.
This monkey likes to wear masks and scare people. He has a collection of over 100 masks!
If your opponent activates a Spell/Trap Card or effect, this card is unmasked. If this card was unmasked, it can make a second attack during each Battle Phase, also if it was unmasked by its own effect, it gains 500 ATK.
If this card is Flip Summoned: You can change its battle position, and if you do, mask it. If this card is unmasked during the Damage Step: It gains 1000 DEF until the end of the Damage Step.
When this card is Tribute Summoned: You can discard 1 card, Special Summon 2 "Papier-mâché Tokens" (Aqua/WATER/Level 1/0 ATK/0 DEF), then mask this card. (Quick Effect): Once per turn: You can banish 2 Mask Monsters from your GY, then target 1 Spell/Trap your opponent controls; unmask this card, and if you do, destroy that target.
If you control 2 or more masked monsters, you can Special Summon this (from your hand). Once per turn: You can target 1 masked monster you control; return it to your hand, and if you do, Special Summon 1 Mask Monster from your hand and mask it.
During the Main Main, the turn player can Normal Summon 1 Mask Monster by masking it, in addition to their Normal Summon/Set. (You can only gain this effect once per turn). Once per turn: You can target 1 masked monster you control; change its battle position. Once per turn, during the End Phase: Each player draws 1 card for each monster they controlled that was unmasked this turn (max. 3).
Target 1 masked monster on the field; unmask that monster. You can banish 2 Mask Monsters from your GY; add this card from your GY to your hand.
Equip only to a Mask Monster. Once per turn, if it unmasks: It gains 1000 ATK until the end of this turn. If the equipped monster would be destroyed by battle or card effect, you can destroy this card instead.
Special Summon 1 Mask Monster from your hand or GY and mask it. You can only activate 1 "Mask Entrance" per turn.
If a Plant monster(s) is sent from your Deck to your GY by a card effect: You can unmask this card, then excavate cards from the top of your Deck equal to the number of Plant monsters sent, send any excavated Plant monsters to the Graveyard, also place the other cards on the bottom of your Deck in any order, also, after that, increase this card's Level by the number of cards sent. If this card is excavated from the Deck and sent to the Graveyard by a card effect: You can place 1 card from your hand on top of your Deck, and if you do, Special Summon this card and mask it. You can only use each effect of "Sylvan Secretsprout" once per turn.
Cannot be Normal Summoned, unless you control a "Ghostrick" monster. Once per turn: You can change this card to face-down Defense Position. If you control another "Ghostrick" monster: You can target 1 "Ghostrick" Spell/Trap in your GY; add it to your hand, and if you do, mask this card. If this card is unmasked during the Damage Step because of an opponent's attack: You can make the ATK of that monster become 0 until the end of the Damage Step.
You can target 1 "Graydle" monster you control; destroy that target, and if you do, Special Summon this card from your hand and mask it. When you activate a "Graydle" Spell/Trap Card, unmask this card. If this card is unmasked, it becomes a Tuner and becomes Level 5. If this card is destroyed by a card effect: You can Special Summon 1 "Graydle" monster from your hand. You can only use each effect of "Graydle Specimen" once per turn.
Red-Eyes Masked Dragon
Once per turn, if exactly 1 "Red-Eyes" monster is Normal or Special Summoned: You can unmask this card, then target 1 "Red-Eyes" monster in your GY whose Level is less than or equal to that monster; Special Summon it in Defense Position. If you activate a "Red-Eyes" Spell/Trap Card: You can mask this card.
Masked Ninja Hikari
If a monster is Special Summoned by the effect of a "Ninjitsu Art" card: You can unmask this card, then target 1 "Ninja" monster in your GY, except "Masked Ninja Hikari"; add it to your hand. If this card is unmasked, it gains 500 ATK and DEF. You can send 1 face-up Continuous Spell/Trap you control to the GY; mask this card, then draw 1 card. You can only use each effect of "Masked Ninja Hikari" once per turn.
Poseidon the Wave Monarch
If this card is Tribute Summoned: You can target up to 2 Spells/Traps on the field; destroy them, and if you do, mask this card. If you have no cards in your Extra Deck (Quick Effect): You can unmask this card, then target 1 monster in your GY with 2400 or more ATK and 1000 DEF; Special Summon that target. You can only use this effect of "Poseidon the Wave Monarch" once per turn. If this Tribute Summoned card is sent to the GY: You can target 1 "Monarch" Spell/Trap in your GY; add it to your hand.