I don't think we'll see slim models for awhile. I mean, it wasn't until 2012 that Sony released the Super Slim PS3, and the original Slim PS3 was born out of necessity, literally dropping hardware in order to decrease size and cost. I think three years is a little optimistic to expect any kind of substantial enough hardware size reduction to turn out a model that is slimmer enough to even warrant calling a slim model. At the very least, Sony won't see it as necessary. Full system revisions like that are usually used when sales are starting to heavily decline, in order to give another push to get momentum going as long as necessary.
As it stands, staggering the top and bottom halves of the system the way they did has left the PS4 not looking all that big to begin with. I mean, it fits on top of my dresser with a 32 inch TV, controllers, and my Vita without any kind of issues, and that's lying down flat. Vertically, the thing is easy to sit in most places without issue because of how flat it is.
