If i do Ethereum Hard Fork to build DApp - Does it need to be on Ethereum or could be a separate blockchain system? If we build a separate blockchain system, what is the advantage of hard-forking from ethereum?
In order to take advantage of the Ethereum ecosystem, you must build your DApp on the main chain. If you were to fork Ethereum, your chain would not have the implicit security and users that the main chain does. Your DApp has to be on the main chain for anyone to (logically) interact with it.