The only constant in the company of Games Workshop is that they worship Tzeentch, the lord of change. Change is always there.
I was a manager candidate at the Baltimore HQ for 3 months a few years ago. I saw big corporate changes three times in that span. They fired Regional managers, then 're organized,' then they fired me and most of my class. This was all in three months.
I am saying this to point out that there is no idea too crazy for GW to try. I believe it is possible for 40k to go the way of AoS, especially the rules for units being in the box. Will it happen? That I can't say as there are too many possibilities.
To say that they are already planning this would be a bit crazy, as they don't even know if AoS is working yet, for the bottom line that is.