[foota]

From [1] "Not every character qualifies for trademark protection, however. For a character to be trademarked, the character cannot be too similar to other existing trademarked characters and must be used to brand products or services. Once a character meets these requirements, the owner can file for trademark protection."

So I don't know if you could apply trademark to e.g., every card in magic, but maybe only to the key characters?

1: https://www.mekiplaw.com/how-to-trademark-a-character-an-eas....