Currently, blockchains are built on three main columns:
One missing part is a unique hashed personality identifier, let's call it "Digital ID". This "Digital ID" is created by scanning a biometric part of your body that is unique to you, hashed, and uploaded to a decentralized blockchain. The Digital-ID Chain would act as Layer 0.
Basically, every transaction on all of the blockchains, either value or code is linked to the personal hashed "Digital ID" -> Layer 0.
Let's say you want to transfer value to person "x", the algorithm could only pursue the transaction if it's validated with the identifier - for example; you need to scan your retina to validate the transaction. This method is better than having some sort of private keys. You can lose your private key or it can get stolen - but you can't lose your eye.
An identifier would make it useless for a bad actor trying to steal someone's private key since transactions can only be pursued and validated with a unique identifier.
If there is a decentralized database with Digital ID's you lower the cost of verification dramatically due to the unique identifier. Someone could verify in REAL-TIME on-site if "x" is the real owner of a house, car, citizenship, or insurance that is saved on the blockchain by only scanning a biometric part of his body and comparing the hash values - person "x" actually doesn't have to reveal any personal information or remember any private key to verify ownership.
Just one use case:
You could travel without any documents.Countries could give a person "x" visa on the blockchain, which is linked to his "Digital-ID". When traveling officers on the border only need to scan the retina of person "x" and compare the hash values. Without person "x" revealing any documents, they would instantaneously know that person "x" is eligible to enter the country. Prevents entering with fake documents or with any document.
Faking documents of ownership gets nearly impossible.
Verification is anonymous.
Verification is instantaneous.
Stealing public/private keys would be useless.
Some problems could be interoperability between different chains, stealing someone's retina (a tendency for crime?), and keeping the Digital-ID Chain alive (what is the incentive for miners?).