Hi @sudipn749,
I would love to see that happen but it is not very likely that they will implement it any time soon.
To have the ability of private messaging is a huge investment. You need a server with all the data stored there, because you can not put all those stuff onto the blockchain.
It can be implemented into Steemit but what about the people using other platforms? Like eSteem or Busy.org for example.
As I see, Steem is too complex and there is many tools out there used to interact with the blockchain to have a universal solution.