If Rockstar really are planning a 2016 announcement at E3, this does not necessarily have to mean that the game itself is going to follow very quickly after that. It could be just an initial reveal and it may not even have any big details about the game.
But it would definitely mean that Rockstar are already far ahead in its development if they are feeling confident enough to showcase the game to the public, so at this point fans bringing up suggestions for what Rockstar could/should do with the title might be a bit too late.
Still, Rockstar is the kind of studio that we would expect to keep a close eye on what their fans are saying in times like these, as these community discussions can be an invaluable source of information about how they should develop the game further, even in terms of more minor details. There are things that can realistically be changed on short notice before the game is released.
There are a few months to go until E3, so we can’t rally so much but wait and see what happens. But if the studio is not actually planning this announcement, they should probably come out and deny the rumors straight away instead of letting them go unaddressed. Otherwise, this could have a negative impact on the reputation of the studio and their game too.