The problem (and the solution, in some cases) is the middlemen. You need to use a well-defined, pre-agreed interface (ERC-20 for tokens, and ERC-721 for NFTs I

The problem (and the solution, in some cases) is the middlemen. You need to use a well-defined, pre-agreed interface (ERC-20 for tokens, and ERC-721 for NFTs I believe) so middlemen like OpenSea can understand and transact your NFT on their platform. If you don't need OpenSea or Rarible, you're free to define them however you like. Also, the reason the "token" is usually a location inside an array in the contract is primarily for gas (and legacy) reasons - storage on-chain is expensive, and it doesn't look like it's getting cheaper anytime soon.