I am going to flake out here and say it will not be one application. Instead, it will be consistent growth across a number of different apps.

If I had to state that one would do it, I will go with "it hasnt been created yet". The reason I say this is because if there is one thing that hooks a massive amount of people, it is a kick-ass game. That is what spreads.

Since we are only starting to see gaming being developed on here, it will take a bit of time. But @steemgg is a start (2 more games already added to it).