In September 2020, after a detailed acquaintance with a number of DeFi projects, I am considering the idea of a possible use of ERC721 tokens. The main point is below.
Being a single indivisible unique entity, the ERC721 token can be associated with an arbitrary set of metadata (Everyone knows an example https://en.wikipedia.org/wiki/CryptoKitties). Nothing prevents us from associating a specific non-fungible token with a specific number of ERC20 tokens. A token issued in this way will have the value of the associated ERC20 tokens and/or the value of the native Ethereum asset.
The minting of such a “secured” ERC721 token must be accompanied by freezing the corresponding amounts of ERC20 and/or ETH tokens on the smart contract balance. The destruction(or burn) of this token, on the contrary, will entail the transfer of “collateral” to the balance of the user who burns the token.