Interoperability

Interoperability happens to be one of the many blockchain trilemma. Interoperability is simply the ability of different blockchain protocols to actively communicate with each other.  
There are different transactions that require interoperability, example;
Crosschain transactions: This is basically an interaction/transaction between just two chains.
Multichain transactions: This is a transaction between three or more chains but with one as a middle man.
Omnichain transactions: This is a transaction between three or more chains but none here serves as a middle man.
Though there's been a number of solutions to the problem of lack of proper interoperability, like sidechains, bridges and oracles, but they haven't solved the problem entirely. For example of recent, bridges have been victims of attacks and users funds were made away with. Bridges do a good job tackling lack of Interoperability but they aren't secure enough. In theory I think in order to get interoperability right, whatever you are bringing as the solution has obey the following rules;
- It has to be a layer0 protocol.
- It has to be an omnichain protocol.
- It has to be very secure.
- It has to be on an Interoperable consensus mechanism like Proof of Time.
If they obey these, I believe we would achieve a level of interoperability that can even enable the movement of NFTs accross different chains.