The collateral can be any other asset that is represented by a cryptographic token. Right now, few crypto-assets map to real world assets in some capacity, but we're willing to make a bet that this will change faster than most expect.
Already, though, there are many interesting assets in the world of crypto that are particularly well suited to being put up for collateral -- namely, the emerging class of crypto-collectibles such as CryptoKitties.
Already, though, there are many interesting assets in the world of crypto that are particularly well suited to being put up for collateral -- namely, the emerging class of crypto-collectibles such as CryptoKitties.