Throughout the Yu-Gi-Oh! anime there have been plenty of cards used by the protagonists that simply aren't real. These cards were made for the anime, and more often than not, are created in order to fulfill some sort of specific purpose. Most of these cards, though, could easily come over to the real-life game.
Some would be good, some would be bad, but no matter how often they would be played, these cards could come over to the real-life game with little to no issues. Sometimes they'd bring quite a few other cards with them.
Ancient Dragon
Used by Solomon Muto during the KC Grand Championship arc of the show, Ancient Dragon is a wildly unnecessarily difficult card to summon. It requires all sorts of other cards and specific requirements to be fulfilled in order to make it onto the field.
In the end, it's a somewhat beefy monster that can destroy defense position cards, that position that no monsters ever go into. It could easily come to the real game.
The Armor Cards
Valon's armor cards existed really just so he and Joey could physically go at it for an entire duel, but they could theoretically exist in the real game.
Sure, they wouldn't be able to attach to the duelist in real life, and would thus lose quite a bit of their appeal, but nevertheless, none of their effects are too wild to make it into the game. It would be a generally smooth transition.
Clone Dragon
Used by Kaiba because it has the word dragon in its title, Clone Dragon has the ability to take the form of another monster that was just summoned to the field. It takes that card's attack and defense points, too.
It's unclear whether it takes effects as well, but it does have a check that requires the user to pay 1000 attack points in order to declare an attack, so it does have some balances built-in in case it takes the form of a beefy monster. This card could be useful in actual gameplay.
Dragon Lady
Vivian Wong's Dragon Lady is a card that requires a spell card and two very specific monsters in order to summon, but once she's on the field, she can destroy a spell or trap card once per turn, and can't attack for that turn if she activates that effect.
That's an okay effect, but the cost is a bit much. Other than that ability, Dragon Lady is a regular monster with 2500 attack points to her name. It would be easy to add her to a real deck.
Five Star Twilight
Five Star Twilight requires a sacrifice of one level five monster. And then, the fun begins when the player get a whole horde of Kuriboh, all in their own fun little colors.
These creatures are all different monsters, but they serve a very specific purpose. They can't be used as tributes to summon other monsters, but they can all be removed from play in order to summon the next card on this list, Kuribabylon. They would make a good addition to a real player's deck.
Kuribabylon
Kuribabylon is a fusion of all of the five Kuriboh brothers. Kuribabylon has the statline of all of the Kuriboh brothers combined, adding up to 1500 attack points.
This card literally only exists to summon the Kuriboh brothers back to the field. This card could very easily make it into the real-life card game, but it might not be the most useful card of all.
Goddess Erda's Guidance
Goddess Erda's Guidance would work fine in the real-life card game, it's just a bit specific, like so many other cards on this list. Its effect is simple, the player can discard one spell card in order to set a trap card from their graveyard onto the field.
It would be pretty easy to play around because the opponent would know which card was getting set. It wouldn't see much play, but it would be easy to turn into a real card.
Magical Trick Mirror
Magical Trick Mirror is another Kaiba classic. The card can only be used in response to an opponent's monster attacking. From there, the player can choose one spell card to activate from their opponent's graveyard.
This could help turn the tide of a battle, or be a complete dud. Either way, it would be a fun one to see implemented into the real-life card game.
Royal Straight Slasher
Royal Straight Slasher can only be summoned by sacrificing Jack's Knight, King's Knight, and Queen's Knight. The card is an effect monster with 2400 attack points.
Its effect allows the player to sacrifice monsters through level 1-5 in order to destroy every card on the opponent's side on the field. This is a wildly expensive cost, but that happens sometimes with real and fake cards.
Little Red Riding Hood
Little Red Riding Hood is a fairytale classic used by Leon in the anime. When the card is destroyed, the player can bring a level-four or lower warrior monster out from their deck.
A solid effect that would be very easy to bring over to the real-life card game. Fans would certainly love to see it.