Check out https://www.steemfiles.com/hive-102930/@tobetada/introducing-a-like-button-to-hive .
There is KYC everyone or use money to prevent sybil attacks.
There are certain changes a front-end maker can do, and certain changes that a front-end maker cannot do. We cannot change the way the blockchain rewards content. The trending page is something that is on the blockchain consensus so this is the same everywhere but a front-end could have a parallel kind of system that uses only what its users does and not the blockchain (for likes). This way your like on one site wouldn't be available through the full-nodes to another but they could conceivably be made available through a public API or perhaps even another blockchain.